Science Reference Sheet

The skull protects the brain. The sternum is the breastbone protecting the front of the chest, not the brain.

The clavicle is the collarbone. The tibia is the shinbone — the large inner bone of the lower leg.

The scapula is the shoulder blade. The clavicle is the collarbone.

The humerus is the single bone of the upper arm, running from the shoulder to the elbow. The femur is the thighbone — the longest, heaviest, and strongest bone in the human body.

The radius is the forearm bone on the thumb side. The ulna is the forearm bone on the pinky side.

The ulna is the forearm bone on the pinky side. The radius is the forearm bone on the thumb side.

The femur is the thighbone — the longest, heaviest, and strongest bone in the human body. The humerus is the single bone of the upper arm, running from the shoulder to the elbow.

The tibia is the shinbone — the large inner bone of the lower leg. The thighbone is the femur in the upper leg, not the lower-leg shinbone.

The fibula is the thin bone on the outer side of the lower leg, running parallel to the tibia. It bears very little body weight but is important for ankle stability and muscle attachment.

The patella is the kneecap, a small bone at the front of the knee. The elbow, ankle, and wrist are different joints.

The pelvis is a basin-shaped ring of bones that supports the upper body's weight and connects the spine to the legs. The femur is the thighbone.

The sternum is the breastbone. The clavicle is the collarbone.

Humans have 12 pairs of ribs, making 24 ribs total. All 12 pairs connect to the spine in the back.

The individual bones of the spine are called vertebrae. The spinal column protects the spinal cord while also allowing flexible movement.

Adults have 206 bones; babies have about 270 that later fuse. 186 misses the adult count.

The stapes, or stirrup bone, is the smallest bone in the human body at only about 3 mm long. The patella is the kneecap, not the tiny stirrup bone in the ear.

The three types of muscle tissue are skeletal, cardiac, and smooth. Skeletal muscle is attached to bones and is under voluntary control.

Skeletal muscle is the type attached to bones by tendons. Cardiac muscle is found only in the walls of the heart.

Cardiac muscle is found only in the walls of the heart. Skeletal muscle is the type attached to bones by tendons.

Smooth muscle lines the walls of hollow organs like the stomach, intestines, and blood vessels. Skeletal muscle is the type attached to bones by tendons.

The biceps bends the elbow, bringing the forearm toward the upper arm. The triceps is on the back of the arm and straightens the elbow.

Quadriceps are four front-thigh muscles that straighten the knee. Hamstrings are back-thigh muscles that bend it.

Hamstrings are back-thigh muscles that bend the knee. Quadriceps sit on the front thigh and straighten it.

Digestive path is mouth, esophagus, stomach, small intestine, large intestine. Reversing or skipping organs breaks the actual food path.

Digestion begins in the mouth. Teeth mechanically break food into smaller pieces while saliva begins chemical digestion.

The esophagus carries food to the stomach by peristalsis. The trachea carries air to the lungs, not food.

The stomach uses acid and enzymes to churn food into a soupy mixture. The liver makes bile and performs many functions, but it is not the acid chamber.

Most nutrient absorption happens in the small intestine, which is about 20 feet long but only about an inch wide. The large intestine is about 5 feet long and 2-3 inches wide.

The large intestine is about 5 feet long and 2-3 inches wide. Most nutrient absorption happens in the small intestine, which is about 20 feet long but only about an inch wide.

The liver makes bile and processes nutrients and toxins. The pancreas makes digestive enzymes and hormones, not bile.

The pancreas produces digestive enzymes that break down proteins, fats, and carbohydrates, and it makes insulin. The liver makes bile and filters blood.

The human heart has 4 chambers: two upper chambers called atria and two lower chambers called ventricles. The left side has 4 H and 2 O atoms.

Arteries carry blood away from the heart. Veins carry blood toward the heart.

Veins carry blood toward the heart. Arteries carry blood away from the heart.

Capillaries are the tiniest blood vessels — so small that red blood cells pass through them single file. Arteries carry blood away from the heart.

Red blood cells carry oxygen from the lungs to every cell in the body. White blood cells are the soldiers of the immune system.

White blood cells are the soldiers of the immune system. Red blood cells carry oxygen from the lungs to every cell in the body.

Platelets are tiny cell fragments that rush to the site of a wound and clump together to form a plug. Red blood cells carry oxygen from the lungs to every cell in the body.

Air enters through the nose, passes down the trachea, branches into the bronchi. Food travels through the digestive system in this order: mouth, esophagus, stomach, small intestine, and large intestine.

The trachea is the windpipe — the tube that carries air from the throat down to the lungs. The open part of the C faces backward toward the esophagus, allowing it to flex slightly when you.

The bronchi are the two tubes that split from the bottom of the trachea, one entering the right lung and. Alveoli are the microscopic, grape-like air sacs at the end of the bronchioles in the lungs.

The diaphragm is a dome-shaped muscle that separates the chest from the abdomen and is the primary muscle of breathing. Intercostal muscles help move the ribs, but the diaphragm is the main breathing muscle.

Alveoli are tiny lung air sacs where oxygen and carbon dioxide exchange. Bronchi are larger air tubes, not the exchange sacs.

We breathe in oxygen, which makes up about 21% of the air around us. We breathe out carbon dioxide.

We breathe out carbon dioxide. We breathe in oxygen, which makes up about 21% of the air around us.

Cerebrum handles thinking, memory, and voluntary movement. Cerebellum coordinates balance and fine movement, not higher thought.

Cerebellum coordinates balance, posture, and fine movement. Cerebrum handles higher thought and voluntary control.

The brain stem connects the brain to the spinal cord and controls the body's most essential involuntary functions. The cerebrum is the largest part of the brain, making up about 85% of its total weight.

A neuron is a nerve cell — the basic building block of the nervous system. A nephron filters blood in the kidney; it is not a nervous-system cell.

Blood returns from the body to the right atrium, moves to the right ventricle, goes to the lungs, then returns left and pumps to the body.

ABO blood types are A, B, AB, and O. C and D are not ABO blood-type labels.

Arteries carry blood away from the heart. Veins carry blood toward the heart.

The gallbladder is a small, pear-shaped sac tucked beneath the liver that stores and concentrates bile. Bile does not digest fat chemically.

Villi are tiny, finger-like projections that line the inner wall of the small intestine. Alveoli are the microscopic, grape-like air sacs at the end of the bronchioles in the lungs.

The spinal cord is the long bundle of nerve tissue running down the center of the vertebral column from the. The brain stem connects the brain to the spinal cord and controls the body's most essential involuntary functions.

CNS and PNS are the two main nervous-system divisions. Voluntary and involuntary pathways are subdivisions, not the top-level split.

Gas exchange takes place in the alveoli — the tiny, grape-like air sacs at the ends of the bronchioles deep in the lungs. Both the alveolar walls and the capillary walls are only one cell thick.

When you inhale, the diaphragm — the dome-shaped muscle below the lungs — contracts and flattens downward. This increases the volume of the chest cavity, which decreases air pressure inside the lungs.

The aorta is the largest artery in the human body, about the diameter of a garden hose. The vena cava is a large vein returning blood; the aorta is the largest artery leaving the heart.

Muscles pull on bones at joints, so the skeletal and muscular systems work together to move the arm.

The nervous system carries signals quickly and helps coordinate reflexes that protect the body.

The nervous system sends fast signals through nerves so your body can respond quickly to danger.

The eye detects light and sends visual information through the optic nerve to the brain.

The digestive system breaks food into nutrients the body can absorb and use.

The excretory system removes wastes from the body. The kidneys filter blood and help produce urine.

The respiratory system brings oxygen into the body through breathing and removes carbon dioxide.

The excretory system removes liquid wastes from the body. The kidneys filter blood, make urine, and send it toward the bladder.

The skeletal system supports the body and protects organs. The rib cage helps protect the heart and lungs.

The muscular system moves the body by contracting. Muscles such as the biceps pull on bones so joints can bend.

The Earth has four layers: the crust, the mantle, the outer core, and the inner core. The inner core is a solid ball of iron and nickel at the very center of Earth.

The Earth has four layers: the crust, the mantle, the outer core, and the inner core. The mantle is the thickest layer of Earth, making up about 84% of Earth's total volume.

The mantle is the thickest layer of Earth, making up about 84% of Earth's total volume. The Earth has four layers: the crust, the mantle, the outer core, and the inner core.

The outer core is a layer of liquid iron and nickel surrounding the solid inner core. The inner core is a solid ball of iron and nickel at the very center of Earth.

The inner core is a solid ball of iron and nickel at the very center of Earth. The outer core is a layer of liquid iron and nickel surrounding the solid inner core.

Igneous rock forms when molten rock cools and solidifies. Sedimentary rock forms when particles of sediment are deposited in layers, usually by water.

Sedimentary rock forms when particles of sediment are deposited in layers, usually by water. Igneous rock forms when molten rock cools and solidifies.

Metamorphic rock forms when heat and pressure change existing rock. Igneous rock forms from cooled molten rock, not from transformed older rock.

The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection. Condensation is the process by which water vapor cools and turns back into liquid water droplets.

The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection. Condensation is the process by which water vapor cools and turns back into liquid water droplets.

Condensation is the process by which water vapor cools and turns back into liquid water droplets. The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection.

Precipitation is any form of water that falls from clouds to the ground — including rain, snow, sleet, and hail. The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection.

The troposphere is the lowest layer of Earth's atmosphere, extending from the surface to about 7-12 miles up. The stratosphere is the second layer of Earth's atmosphere, extending from about 12 to 31 miles above the surface.

The five layers of Earth's atmosphere from lowest to highest are: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.

The stratosphere is the second layer of Earth's atmosphere, extending from about 12 to 31 miles above the surface. The troposphere is the lowest layer of Earth's atmosphere, extending from the surface to about 7-12 miles up.

The rock cycle is the continuous process by which rocks are transformed from one type to another over long periods of time. There is no beginning or end to the cycle.

Mohs scale ranks scratch hardness from talc 1 to diamond 10. Density or color tests do not measure scratch resistance.

Igneous rocks form when magma or lava cools and hardens.

Sedimentary rocks form when sediment layers are compacted and cemented together.

Metamorphic rocks form when heat and pressure change existing rock.

Hardness is a mineral property. The Mohs scale compares how easily minerals scratch one another.

Sedimentary rocks are most likely to contain fossils because they form from layers of deposited sediment.

Climate describes the usual weather pattern of a region over a long time.

A front is a boundary where different air masses meet, often bringing weather changes.

Dark clouds and falling air pressure can signal rain or stormy weather.

Climate is a long-term regional pattern; one desert storm is weather. One event cannot reset a yearly climate average.

When a fast-moving cold air mass pushes underneath a warmer air mass, it forces the warm air to rise quickly. This rapid lifting often creates fast-moving, sometimes strong storms — the kind of weather typical of a cold front passing through.

The outer core is a layer of liquid metal. Its motion helps generate Earth's magnetic field.

Evaporation changes liquid water into water vapor, which explains why a puddle can disappear in warm sunlight.

The inner core is the hottest, deepest layer of Earth. It is mostly solid iron and nickel under enormous pressure.

The outer core is liquid iron and nickel. Its motion helps produce Earth's magnetic field.

From outside to inside, Earth's layers are crust, mantle, outer core, and inner core.

Collection is when water gathers in oceans, lakes, rivers, ice, soil, or underground after precipitation or runoff.

Runoff is water that flows over the land surface after rain or melting snow, eventually reaching streams, rivers, lakes, or the ocean.

Transpiration is when plants release water vapor from their leaves into the air. It is part of the water cycle.

A simple water cycle path is evaporation, condensation, precipitation, and collection. Water then repeats the cycle.

Infiltration is when water soaks into the ground, becoming soil moisture or groundwater.

Igneous rock forms when molten rock cools and hardens.

Sedimentary rock forms when layers of sediment are compacted and cemented together.

Metamorphic rock forms when existing rock is changed by heat and pressure.

Hardness tells how resistant a mineral is to scratching.

Evaporation is when liquid water changes into water vapor.

Condensation is when water vapor cools and changes into tiny liquid droplets, forming clouds.

Precipitation is water falling from clouds as rain, snow, sleet, or hail.

The crust is Earth's thin, rocky outer layer.

The mantle lies below the crust and is made of very hot rock that can move slowly over time.

Earth's core is made mostly of iron and nickel.

Evaporation changes liquid water into water vapor, which is why a puddle can disappear on a sunny day.

The crust is Earth's thin, rocky outer layer. It is broken into tectonic plates that carry continents and ocean floors.

The mantle is the hot rock layer beneath the crust. Slow movement in the mantle helps drive the motion of tectonic plates.

Precipitation happens when water falls from clouds to Earth as rain, snow, sleet, or hail.

The animal kingdom includes all multicellular organisms that must consume other organisms for energy. The plant kingdom includes all multicellular organisms that make their own food through photosynthesis.

The plant kingdom includes all multicellular organisms that make their own food through photosynthesis. The animal kingdom includes all multicellular organisms that must consume other organisms for energy.

The fungi kingdom includes mushrooms, molds, and yeasts. The plant kingdom includes all multicellular organisms that make their own food through photosynthesis.

The three main parts of a plant are roots, stem, and leaves. Many plants also have flowers and fruits.

Roots absorb water and dissolved minerals from the soil through tiny root hairs and anchor the plant so it does not fall. Some roots also store food — carrots, turnips, and sweet potatoes are actually storage roots we eat.

Stems support plants and transport water and nutrients between roots and leaves. Photosynthesis mainly happens in leaves, not stems.

Leaves are the food-making factories of the plant. The broad, flat shape of most leaves maximizes the surface area available to absorb sunlight.

Photosynthesis is the process plants use to make their own food. Using sunlight as energy, they combine water and carbon dioxide to produce glucose and oxygen.

Nucleus stores DNA and directs cell activities. Cell membrane controls entry and exit at the boundary, not genetic control.

Cell wall is a rigid plant-cell layer for support. Cell membrane is a flexible boundary in all cells, not the rigid wall.

Mitochondria are the powerhouses of the cell. Chloroplasts are organelles found in plant cells that capture sunlight and use it to power photosynthesis.

Chloroplasts are organelles found in plant cells that capture sunlight and use it to power photosynthesis. Mitochondria are the powerhouses of the cell.

A vacuole is a storage organelle inside the cell. The nucleus is the control center of the cell, containing the cell's DNA.

A vertebrate is an animal with a backbone. An invertebrate is an animal without a backbone.

An invertebrate is an animal without a backbone. A vertebrate is an animal with a backbone.

Mammals are warm-blooded vertebrates with hair or fur that feed milk to young. Reptiles are cold-blooded, scaly, and usually lay eggs on land.

Reptiles have dry, scaly skin and usually lay eggs on land. Amphibians typically begin life in water and later live partly on land.

Amphibians usually live a double life: water-dwelling young and land-capable adults. Reptiles have dry scales and lay eggs on land.

The five kingdoms of life are: Monera, Protista, Fungi, Plantae, and Animalia. This five-kingdom system was widely used in the 20th century.

The cell membrane is the thin, flexible boundary that surrounds every cell, separating the interior of the cell from its. The cell wall is a rigid outer layer found in plant cells but not in animal cells.

Vertebrate classes are fish, amphibians, reptiles, birds, and mammals. Invertebrate groups lack the backbone criterion.

A habitat is the natural environment where an organism lives. A biome is a broad regional habitat; an ecosystem is local organisms plus nonliving factors.

Metamorphosis is the biological process of dramatic transformation in body form and function during an organism's life cycle. Germination is seed sprouting; metamorphosis is body-form change during development.

Five traditional senses are sight, hearing, smell, taste, and touch. Balance or pain are additional senses, not the traditional five.

The three main macronutrients — substances the body needs in large amounts — are carbohydrates, proteins, and fats. Fats provide concentrated energy, insulate organs, and are essential for hormone production and absorbing fat-soluble vitamins.

A butterfly life cycle begins with an egg laid on a host plant, which then hatches into a larva (caterpillar).

A caterpillar is the larva stage of a butterfly.

The pupa stage is when the caterpillar changes into an adult butterfly inside the chrysalis.

A simple frog life cycle is egg, tadpole, then adult frog.

Butterflies have complete metamorphosis: egg, larva, pupa, adult.

A nymph is a young stage in incomplete metamorphosis. It resembles the adult but is not fully mature.

Mammals are vertebrates that usually have hair or fur and feed milk to their young.

Reptiles are cold-blooded vertebrates that commonly have scales and lay eggs on land.

The stem supports the plant and helps transport water from the roots to the leaves.

Roots absorb water and nutrients from the soil, so weak or damaged roots can cause wilting.

A butterfly has complete metamorphosis: egg, larva, pupa, and adult.

A tadpole is the aquatic young stage of a frog.

Most mammals give live birth and nurse their young with milk.

Tadpole and frog are stages in one animal life cycle.

The stem supports the plant and helps move water and nutrients between roots and leaves.

Birds are warm-blooded vertebrates with feathers. Most lay eggs, so feathers plus egg-laying point to the bird group.

Tundra is extremely cold, dry, and treeless, with permafrost below the surface. Taiga is a cold conifer forest south of much tundra.

The taiga is the largest land biome on Earth, stretching across northern Canada, Russia, and Scandinavia. Tundra is treeless permafrost; taiga is the northern conifer forest.

The temperate deciduous forest biome has four distinct seasons. Tropical rainforests are found near the equator and receive over 80 inches of rain per year with warm temperatures year-round.

Grasslands are biomes dominated by grasses with few or no trees, receiving moderate rainfall. A desert is defined by its dryness — receiving less than 10 inches of precipitation per year — not by its temperature.

A desert is defined by its dryness — receiving less than 10 inches of precipitation per year — not by its temperature. The tundra biome is extremely cold, receiving little precipitation and having a permanently frozen layer of soil called permafrost just.

Tropical rainforests are found near the equator and receive over 80 inches of rain per year with warm temperatures year-round. The temperate deciduous forest biome has four distinct seasons.

Taxonomy order is Kingdom, Phylum, Class, Order, Family, Genus, Species. A reversed or scrambled order loses broad-to-specific ranking.

King Philip Came Over For Good Spaghetti maps Kingdom, Phylum, Class, Order, Family, Genus, Species. Kings Play Cards On Fat Green Stools scrambles that order.

A food chain shows the linear sequence of who eats whom in an ecosystem. The water cycle tracks water movement, not feeding relationships.

A producer is an organism that makes its own food using energy from the sun through photosynthesis. A consumer is an organism that must eat other organisms to get energy because it cannot make its own food.

A consumer is an organism that must eat other organisms to get energy because it cannot make its own food. A producer is an organism that makes its own food using energy from the sun through photosynthesis.

Decomposers break down dead organisms and organic waste, returning nutrients like nitrogen and carbon to the soil so plants can. A producer is an organism that makes its own food using energy from the sun through photosynthesis.

An herbivore is an animal that eats only plants. A carnivore is an animal that eats only other animals.

A carnivore is an animal that eats only other animals. An herbivore is an animal that eats only plants.

An omnivore eats both plants and animals, giving it a flexible diet. An herbivore eats only plants.

An ecosystem is all the living organisms in an area interacting with each other and with the nonliving components. A biome is a broad regional habitat; an ecosystem is local organisms plus nonliving factors.

A food web is a map of all the feeding relationships in an ecosystem — a set of interconnected food chains. A food chain shows the linear sequence of who eats whom in an ecosystem.

Symbiosis is any close, long-term interaction between two different species. Competition is organisms seeking the same resource, not a close long-term species interaction.

Mutualism benefits both species in a relationship. Commensalism benefits one species while the other is unaffected.

An adaptation is a heritable trait that helps an organism survive and reproduce in its specific environment. A mutation is a DNA change; adaptation is a heritable helpful trait shaped over generations.

Camels have adaptations that help them survive in desert habitats.

Grass is the producer because it makes its own food through photosynthesis. The rabbit and fox are consumers.

Thick fur and fat help a polar bear conserve heat in an Arctic or polar habitat.

A forest habitat has many trees, which provide food and shelter for animals such as squirrels.

Deserts are dry habitats, and many desert plants store or conserve water.

Thick fur helps conserve heat, so it is useful in cold biomes such as tundra.

Grass fills the producer role because plants make their own food by photosynthesis. The grasshopper and frog are consumers.

Decomposers such as fungi break down dead material and return nutrients to the ecosystem.

Planet order from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Reversing it starts farthest-out, not nearest.

The 8 planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are called gas giants and ice giants.

The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are called gas giants and ice giants. The 8 planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Jupiter is the largest planet in our solar system. Saturn is famous for rings, but it is smaller than Jupiter.

Mercury is the smallest planet and the closest to the Sun. Pluto is smaller, but it is classified as a dwarf planet.

Saturn is famous for its spectacular ring system. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are called gas giants and ice giants.

Our Sun is a yellow dwarf star. A red giant is an expanded late-life star, not the Sun's current yellow-dwarf stage.

Our galaxy is the Milky Way. Andromeda is another large nearby galaxy, not the galaxy that contains our solar system.

A new moon occurs when the Moon is positioned between the Earth and the Sun. A full moon occurs when the Earth is positioned between the Sun and the Moon.

A full moon occurs when the Earth is positioned between the Sun and the Moon. A new moon occurs when the Moon is positioned between the Earth and the Sun.

Moon phases progress new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent. Reversing breaks waxing/waning order.

Waxing means the Moon's lit portion is growing from new toward full. Waning means the lit portion is shrinking after full.

Waning means the Moon's lit portion is shrinking from full toward new. Waxing means the lit portion is growing.

Earth rotates once on its axis approximately every 24 hours, which is why we have a day-and-night cycle. As Earth spins, different parts of its surface face toward the Sun or away from it.

Earth revolves around the Sun in about 365.25 days. Rotation takes about 24 hours and causes day and night.

Mars is called the Red Planet because iron oxide on its surface gives it a rusty color. Venus is hotter, but it is not the Red Planet.

Venus is the hottest planet in our solar system with surface temperatures reaching 465°C — hot enough to melt lead. The 8 planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The asteroid belt is a region between the orbits of Mars and Jupiter containing millions of rocky and metallic objects. The asteroid belt lies outside Mars, not between Earth and Mars.

A light-year is the distance light travels in one year, about 5.88 trillion miles. Despite the word year, it measures distance, not time.

A constellation is a pattern or grouping of stars as seen from Earth, often given a name based on what the pattern. The word comes from the Latin constellatio, meaning group of stars.

Ocean tides are primarily caused by the Moon's gravitational pull, which stretches Earth's oceans slightly toward the Moon. Simultaneously, a second bulge forms on the opposite side of Earth because inertia causes the water there to '.

Spring tides occur when the Sun, Moon, and Earth are aligned in a straight line — during new moon and full moon. When all three are aligned, the Sun's gravitational pull reinforces the Moon's, creating exceptionally high tides and very.

The 8 planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Venus is the hottest planet in our solar system with surface temperatures reaching 465°C — hot enough to melt lead.

Neptune is the eighth and farthest planet from the Sun and takes about 165 Earth years to orbit once. Uranus is seventh.

During a first quarter moon, exactly half of the Moon's visible face is illuminated — the right half. This is one of the most confusing facts in lunar science: the name ' quarter ' refers to the Moon.

Around midday, the Sun is usually highest in the sky, so objects cast shorter shadows than they do in the morning or evening.

A galaxy is an enormous system containing many stars along with gas, dust, and dark matter, all held together by gravity.

Our solar system is located in the Milky Way galaxy, a large spiral galaxy containing the Sun and many other stars.

The universe is the largest choice because it includes all galaxies, stars, planets, matter, and space.

A light-year is a unit of distance: the distance light travels in one year. It is useful because space is extremely large.

Andromeda is a galaxy near the Milky Way. It contains many stars, not just one planet or moon.

Seeing many distant galaxies shows that the universe contains far more than the Milky Way and our solar system.

Stars are hot, glowing balls of gas that produce their own light. Planets and moons are visible because they reflect light.

A constellation is a named star pattern or sky region. Constellations help people identify parts of the night sky.

Polaris, the North Star, appears near the north celestial pole and can help observers in the Northern Hemisphere find north.

Stars appear to move because Earth rotates. The stars are not circling Earth each night; our viewpoint is turning.

Star color gives a clue about temperature. Blue or blue-white stars are generally hotter than red stars.

The Big Dipper is a familiar star pattern within Ursa Major. It is useful for finding other stars, including Polaris.

The Sun is a star. It is the closest star to Earth, which is why it looks so big and bright compared to the other stars we see at night.

There are 8 planets in our solar system. Pluto used to be counted as the 9th, but it is now called a dwarf planet.

Jupiter is the largest planet in our solar system. It is so big that all the other planets could fit inside it.

Saturn is famous for its bright rings made of ice and rock. Some other planets have faint rings too, but Saturn's are the largest and easiest to see.

The Sun gives Earth light and heat. Its light lets us see and helps plants grow, and its heat keeps our planet warm enough for living things.

A planet's orbit is the path it follows as it revolves around the Sun. This is different from a spin, which is when a planet turns around in place.

To rotate means to spin around in place. Earth rotates once a day. This is different from revolving, which means traveling all the way around the Sun.

The asteroid belt is a ring of rocky chunks called asteroids that orbit the Sun between Mars and Jupiter. It is not the same as the Milky Way, which is our whole galaxy of stars.

A meteor is the bright streak we see when a small space rock burns up in Earth's air. A comet is different: it is a ball of ice and dust with a long glowing tail.

Pluto is called a dwarf planet. It is not a moon, which orbits a planet instead of the Sun.

Saturn has the most moons of any planet, with well over a hundred. Earth has only one Moon, so a planet can have many moons or just a few.

Venus is the hottest planet. Its thick clouds trap heat and hold it in. Mercury is closer to the Sun, but it has almost no air to keep heat from escaping, so Venus stays hotter.

Mercury, Venus, Earth, and Mars are the inner rocky planets. They are small and solid. The outer gas giants, like Jupiter and Saturn, are much larger and made mostly of gas.

Earth spins, or rotates, on its axis once about every 24 hours. The side facing the Sun has daytime, and the side turned away has nighttime.

Earth travels all the way around the Sun once about every 365 days, which is one year. This long journey is called Earth's revolution.

Because Earth spins toward the east, the Sun appears to rise in the east each morning and set in the west each evening.

The Moon does not make its own light. It shines because sunlight hits its surface and bounces, or reflects, back toward Earth.

The Moon is Earth's natural satellite. It orbits, or travels around, Earth about once every month.

During a new moon, the side of the Moon lit by the Sun faces away from Earth, so we cannot see the bright part of the Moon in the sky.

Earth is tilted on its axis. As Earth travels around the Sun, the tilt makes one part of Earth lean toward the Sun for summer and away for winter. It is not about getting closer or farther.

The Sun looks like it crosses the sky, but it is really Earth that is rotating. As we spin, our view of the Sun slowly changes from morning to evening.

The Moon's gravity gently pulls on Earth's oceans. This pull is the main cause of high and low tides along the coast each day.

During a solar eclipse, the Moon passes directly between the Sun and Earth, blocking the Sun's light and casting a shadow on part of Earth.

When the bright part of the Moon grows larger night after night, the Moon is waxing. When it shrinks back down, it is waning.

In a lunar eclipse, Earth moves between the Sun and the Moon. Earth's shadow then falls on the Moon, making it look dark or reddish.

A star is a giant ball of hot, glowing gas that gives off its own light. It is not solid like a rocky planet.

The Sun is the closest star to Earth. It is a regular star that looks much bigger and brighter than the others because it is so much closer to us.

Stars look like they twinkle because their light passes through Earth's moving air, which bends the light a little. The stars themselves shine steadily.

Polaris, the North Star, is the bright star at the end of the handle of the Little Dipper. The nearby Big Dipper can help you find it.

Sirius is the brightest star in the night sky as seen from Earth. It is part of the constellation Canis Major and is nicknamed the Dog Star.

If two stars give off the same light, the one that is closer to Earth looks brighter to us. How bright a star looks depends on both its real brightness and its distance.

A star's color is a clue to its temperature. Reddish stars are generally cooler, while blue-white stars are generally hotter.

A galaxy is a huge group of stars, gas, and dust that are all held together by gravity, the same invisible pull that keeps your feet on the ground.

Many galaxies, including our own Milky Way, are spiral galaxies. They have curving arms that wind outward from the center, a little like a pinwheel.

A nebula is a giant cloud of gas and dust floating in space. Some nebulae glow softly, which is why we can see them through telescopes.

The universe is vastly larger than our solar system. It contains countless galaxies, and each galaxy holds many stars and solar systems like ours.

Telescopes collect much more light than our eyes can, so faint and very distant objects like galaxies and nebulae appear brighter and clearer.

A galaxy is far bigger than a solar system. A solar system is one star with its planets, while a galaxy holds billions of stars and many solar systems like ours.

A light-year is a measure of distance, the distance light travels in one year. Saying a galaxy is 100,000 light-years across tells you how far it stretches from one side to the other.

As Earth rotates, the Sun appears to move across the sky. When the Sun is higher, shadows are shorter.

A full Moon occurs when Earth is generally between the Sun and the Moon, so the Moon's near side is fully illuminated.

A lever is a rigid bar that pivots around a fixed point called the fulcrum. A pulley is a wheel with a groove that holds a rope or chain, used to lift or move loads.

A wheel and axle is a simple machine consisting of a large wheel attached to a smaller rod. An inclined plane is a flat surface tilted at an angle — basically a ramp.

A pulley is a wheel with a groove that holds a rope or chain, used to lift or move loads. A lever is a rigid bar that pivots around a fixed point called the fulcrum.

An inclined plane is a flat surface tilted at an angle — basically a ramp. A wheel and axle is a simple machine consisting of a large wheel attached to a smaller rod.

A wedge is a simple machine formed by two inclined planes joined back-to-back, tapering to a thin edge. An inclined plane is a flat surface tilted at an angle — basically a ramp.

A screw is an inclined plane wrapped in a spiral around a cylinder. A lever is a rigid bar that pivots around a fixed point called the fulcrum.

The main types of energy are kinetic, potential, thermal, chemical, electrical, nuclear, light, and sound. The four states of matter are solid, liquid, gas, and plasma.

Kinetic energy is the energy of motion — any object that is moving has kinetic energy. Potential energy is stored energy based on an object's position or condition.

Potential energy is stored energy based on an object's position or condition. Kinetic energy is the energy of motion — any object that is moving has kinetic energy.

Thermal energy is the total kinetic energy of all the particles in an object. Chemical energy is stored in the chemical bonds between atoms in molecules.

Chemical energy is stored in the chemical bonds between atoms in molecules. Nuclear energy comes from atomic nuclei, not chemical bonds between atoms.

Common states are solid, liquid, gas, and plasma. Atoms, molecules, and compounds are particle/substance categories, not states.

The four states of matter are solid, liquid, gas, and plasma. In a liquid, particles are close together but can slide past each other.

In a liquid, particles are close together but can slide past each other. The four states of matter are solid, liquid, gas, and plasma.

Gas particles spread far apart to fill a container. Liquid particles stay close and keep volume.

Plasma is ionized gas with charged particles. Ordinary gas usually has neutral particles, so stars are plasma, not merely gas.

Melting is the phase change from solid to liquid when a substance absorbs enough heat energy to overcome the forces. Freezing is the phase change from liquid to solid when a substance loses enough heat energy that its particles slow.

Freezing is the phase change from liquid to solid when a substance loses enough heat energy that its particles slow. Melting is the phase change from solid to liquid when a substance absorbs enough heat energy to overcome the forces.

The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection. Condensation is the process by which water vapor cools and turns back into liquid water droplets.

Condensation is the process by which water vapor cools and turns back into liquid water droplets. The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection.

Sublimation is the phase change from solid directly to gas, skipping the liquid phase. The water cycle continuously moves water through the environment: evaporation, condensation, precipitation, and collection.

Gravity is a fundamental force of attraction between any two objects with mass. Magnetism acts on magnetic materials and moving charges, not all mass.

Friction is the force that resists the sliding motion between two surfaces in contact. Gravity is a fundamental force of attraction between any two objects with mass.

Speed = distance ÷ time. If a car travels 120 miles in 2 hours, its speed is 120 ÷ 2 = 60 mph.

Electrical energy is energy carried by the flow of electrons through a conductor. Chemical energy is stored in the chemical bonds between atoms in molecules.

Sound energy is produced when objects vibrate, sending compression waves through a medium. A lamp transforms electrical energy into light energy, with some heat as well.

Light is energy that travels as electromagnetic waves at approximately 186,000 miles per second — the fastest anything can travel. Sound energy is produced when objects vibrate, sending compression waves through a medium.

The Law of Conservation of Energy states that energy can never be created or destroyed. Newton's First Law describes inertia, not energy changing form.

A book sitting still on a table perfectly illustrates Newton's First Law — the Law of Inertia. The book is at rest and will remain at rest because the net force on it is zero.

When you jump, your legs push down on the ground — that is the action force. Newton's Third Law states that the ground pushes back up on you with an equal and opposite force.

In a Class 1 lever, the fulcrum is located between the effort and the load. In a Class 2 lever, the load is between the fulcrum and effort.

Newton's First Law says motion persists unless a force acts. F=ma is the Second Law, not inertia.

Newton's Second Law of Motion states that force equals mass times acceleration: F = ma. This means: the more force you apply to an object, the more it accelerates.

Newton's Third Law of Motion states: for every action, there is an equal and opposite reaction. Forces always come in pairs.

Gravity is the force of attraction between any two objects that have mass. The greater the mass of the objects and the closer they are, the stronger the gravitational force.

Friction is the force that opposes the motion of surfaces sliding against each other. There are different types: static friction, kinetic friction, and rolling friction.

A lever is a rigid bar pivoting on a fulcrum. An inclined plane is a sloped surface, not a pivoting bar.

When light passes from one medium to another, it changes speed, which causes it to change direction — this is called refraction. Light travels at different speeds in different materials: faster in air, slower in water, even slower in glass.

Sound needs a material medium whose particles vibrate. Light is electromagnetic and can travel through empty space.

Magnets have two poles: North and South. The fundamental rule of magnetism is that opposite poles attract and like poles repel.

A measurement uses a number and a unit, such as 12 centimeters.

A ruler measures length in units such as inches or centimeters.

A balance scale measures mass, often in grams.

A thermometer measures temperature, commonly in degrees Celsius or Fahrenheit.

A graduated cylinder measures liquid volume, often in milliliters.

Grams measure mass. Liters measure volume, degrees measure temperature, and seconds measure time.

Repeating measurements helps reduce mistakes and makes the result more reliable.

Halfway between 40 mL and 50 mL is 45 mL. Reading scales carefully is part of measurement.

A closed circuit has a complete path, so electric current can flow.

An open circuit breaks the path, so current cannot flow through the bulb.

Copper is a good conductor, which is why it is commonly used in electrical wires.

Plastic is an insulator, so it helps protect people and keeps current in the conducting wire.

A battery provides electrical energy that can move charges through a circuit.

Reflection happens when light bounces off a surface such as a mirror.

Refraction is the bending of light as it passes from one material into another.

A prism can separate white light into colors of the spectrum.

An opaque object blocks light, leaving a darker shadow behind it.

Sound begins with vibration. Vibrating objects disturb the surrounding medium.

Sound travels when particles in a medium, such as air, vibrate and pass the energy along.

Faster vibrations have higher frequency, which we hear as higher pitch.

Thermal energy is energy related to heat and particle motion.

A lamp transforms electrical energy into light energy, with some heat as well.

Rough carpet creates more friction because its surface catches and resists sliding motion.

Bicycle brakes use friction to oppose the wheel's motion and slow it down.

A wheelbarrow acts as a lever, with the wheel area as a fulcrum and the handles applying effort.

Mass stays the same, but weight changes with gravity. The Moon has weaker gravity than Earth.

Gravity is the force that pulls objects toward Earth, so it explains why a dropped ball falls.

Gravity pulls objects toward Earth's center. An object's weight is the force of gravity acting on it.

A ramp is an inclined plane. It lets a load move upward with less force over a longer distance.

A transparent material such as clear glass lets light pass straight through. An opaque material blocks light entirely, and a translucent material lets only some light through.

Refraction is the bending of light as it passes from one material into another.

Transparent materials let most light pass through clearly.

A shadow forms when an opaque object blocks light from reaching a surface.

Friction is a force that opposes motion, so it often slows moving objects.

Rough surfaces usually create more friction than smooth surfaces.

Newton's first law says an object changes its motion only when a force acts on it.

A force is a push or pull that can change an object's motion.

An open circuit has a break in the path, so electric current cannot flow all the way around.

Copper is a good conductor, so it lets electric current move through it easily.

Rubber is an insulator because electric current does not pass through it easily.

A bulb lights when current can travel through a complete closed path with an energy source.

Kinetic energy is the energy an object has because it is moving.

Potential energy is stored energy. A stretched bow can release that energy as motion.

Thermal energy is energy related to the motion of particles and is experienced as heat.

A lamp changes electrical energy into light energy and some thermal energy.

Sound is produced by vibrations that travel through matter as waves.

Sound needs matter to carry vibrations. It travels through air, water, and solids, but not through a perfect vacuum.

Higher pitch comes from higher frequency, which means faster vibration.

Louder sounds usually have greater amplitude, meaning larger vibrations.

A doorknob works like a wheel and axle because turning the larger wheel turns the central axle.

A wedge is a simple machine used to split, cut, or push material apart.

A pulley uses a wheel and rope to change the direction of a force and help lift loads.

Gravity pulls the Moon and Earth toward each other and helps keep the Moon in orbit.

More massive objects have stronger gravitational pull.

Alkali metals are the highly reactive metals in Group 1 of the periodic table: lithium, sodium, potassium, rubidium, cesium. Alkaline earth metals are the reactive metals in Group 2 of the periodic table: beryllium, magnesium, calcium, strontium, barium.

Alkaline earth metals are the reactive metals in Group 2 of the periodic table: beryllium, magnesium, calcium, strontium, barium. Alkali metals are the highly reactive metals in Group 1 of the periodic table: lithium, sodium, potassium, rubidium, cesium.

Halogens are the highly reactive nonmetals in Group 17 of the periodic table: fluorine, chlorine, bromine, iodine, and astatine. Noble gases are the unreactive gases in Group 18 of the periodic table: helium, neon, argon, krypton, xenon, and radon.

Noble gases are the unreactive gases in Group 18 of the periodic table: helium, neon, argon, krypton, xenon, and radon. Halogens are the highly reactive nonmetals in Group 17 of the periodic table: fluorine, chlorine, bromine, iodine, and astatine.

Transition metals are the large group of metals that fill the middle section of the periodic table. Alkali metals are the highly reactive metals in Group 1 of the periodic table: lithium, sodium, potassium, rubidium, cesium.

A proton is a positively charged subatomic particle found in the nucleus of every atom. A neutron is a subatomic particle with no electrical charge found in the nucleus alongside protons.

A neutron is a subatomic particle with no electrical charge found in the nucleus alongside protons. A proton is a positively charged subatomic particle found in the nucleus of every atom.

An electron is a negatively charged subatomic particle that exists in shells around the nucleus. A proton is a positively charged subatomic particle found in the nucleus of every atom.

The nucleus is the tiny, dense center of an atom, containing protons and neutrons. Nearly all of the atom's volume is empty space where electrons move.

The atomic number tells you how many protons are in an atom's nucleus. Elements are arranged in the periodic table in order of increasing atomic number.

An ionic bond forms when one atom transfers electrons to another, creating positively and negatively charged ions that are attracted. A covalent bond forms when two atoms share electrons, allowing both atoms to achieve a full outer electron shell.

A covalent bond forms when two atoms share electrons, allowing both atoms to achieve a full outer electron shell. An ionic bond forms when one atom transfers electrons to another, creating positively and negatively charged ions that are attracted.

A metallic bond is the attraction between metal atoms and the 'sea of electrons' that flows freely among them. An ionic bond forms when one atom transfers electrons to another, creating positively and negatively charged ions that are attracted.

An acid is a substance with a pH below 7 that donates hydrogen ions in solution. A base is a substance with a pH above 7 that accepts hydrogen ions in solution.

A base is a substance with a pH above 7 that accepts hydrogen ions in solution. An acid is a substance with a pH below 7 that donates hydrogen ions in solution.

pH 7 is neutral — the pH of pure water. pH 0 is strongly acidic; pH 7 is neutral.

The pH scale runs from 0 to 14. The pH scale normally runs 0 to 14, not 1 to 10.

An element is a pure substance made of only one type of atom. A compound is a substance formed when two or more different elements are chemically bonded together in fixed ratios.

A compound is a substance formed when two or more different elements are chemically bonded together in fixed ratios. An element is a pure substance made of only one type of atom.

A mixture is a combination of two or more substances that are physically combined but NOT chemically bonded. A compound is a substance formed when two or more different elements are chemically bonded together in fixed ratios.

A molecule is the smallest unit of a compound that retains its chemical properties. An element is a pure substance made of only one type of atom.

An ion is an atom or molecule that has gained or lost one or more electrons, giving it a net. A neutral atom has balanced charge; an ion has gained or lost electrons.

Periodic table orders elements by atomic number and groups similar properties. Alphabetical order would hide chemical families.

A chemical reaction is a process in which the atoms of reactants rearrange to form new substances with different properties. A physical change alters the form, size, or state of a substance without changing what it is chemically.

Reactants are the starting substances in a chemical reaction — the ingredients that get transformed. Products are the new substances formed as a result of a chemical reaction.

Products are the new substances formed as a result of a chemical reaction. Reactants are the starting substances in a chemical reaction — the ingredients that get transformed.

The Law of Conservation of Mass states that in any chemical reaction, the total mass of the reactants equals the. The Law of Conservation of Energy states that energy can never be created or destroyed.

A physical change alters the form, size, or state of a substance without changing what it is chemically. A chemical change produces one or more new substances with different chemical properties from the original.

A chemical change produces one or more new substances with different chemical properties from the original. A physical change alters the form, size, or state of a substance without changing what it is chemically.

Periods are the horizontal rows of the periodic table. Groups are the vertical columns of the periodic table, numbered 1-18.

Groups are the vertical columns of the periodic table, numbered 1-18. Periods are the horizontal rows of the periodic table.

Dmitri Mendeleev was the Russian chemist who published the first widely accepted periodic table in 1869, organizing 63 known elements by atomic. What made his work brilliant was that he left gaps for elements not yet discovered and correctly predicted.

Metalloids are elements with properties intermediate between metals and nonmetals. Transition metals are the large group of metals that fill the middle section of the periodic table.

The chemical symbol for oxygen is O. Ox is not oxygen's symbol; oxygen uses the one-letter symbol O.

Hydrogen uses the symbol H; helium uses He. Hydrogen, not helium, is the lightest and most abundant element.

Gold uses the symbol Au from Latin aurum. Go is not a chemical symbol for gold.

Iron uses Fe from Latin ferrum. Ir names iridium, so it cannot stand for iron.

Carbon is C and forms four bonds, enabling chains and rings. Nitrogen is N and follows different bonding patterns.

The chemical symbol for sodium is Na, from the Latin-derived name natrium. So is not a chemical symbol for sodium; sodium is Na from natrium.

Water is H2O: two hydrogens bonded to one oxygen. HO2 reverses the count and is not water.

NaCl pairs sodium with chlorine to make table salt. KCl is potassium chloride, not sodium chloride.

CO2 has one carbon and two oxygen atoms. CO is carbon monoxide with only one oxygen.

Oxygen gas is O2 and supports cellular respiration. CO2 is carbon dioxide, a gas cells release.

An exothermic reaction releases heat energy into the surroundings. An endothermic reaction absorbs heat energy from the surroundings.

An endothermic reaction absorbs heat energy from the surroundings. An exothermic reaction releases heat energy into the surroundings.

A catalyst lowers activation energy and speeds a reaction without being used up. A reactant is consumed or changed by the reaction.

In a solution, the solute is the substance that gets dissolved into the solvent. When you make sweet tea, sugar is the solute and water is the solvent.

Water dissolves more kinds of substances than any other liquid. Alcohol dissolves some substances, but not as broadly as water.

The mass number is the total count of protons plus neutrons in an atom's nucleus. Carbon-12 has 6 protons + 6 neutrons = mass number 12.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, giving. Ions differ by electron gain or loss; isotopes differ by neutron count.

Electron shells are the regions around the nucleus where electrons are found. Orbitals describe probability regions inside shells, not the shell levels themselves.

Valence electrons are the electrons in an atom's outermost shell. Atoms with 8 valence electrons are stable and do not readily react — these are the noble gases.

First five elements are Hydrogen, Helium, Lithium, Beryllium, Boron. Repeating hydrogen alone misses the sequence.

Elements 6-10 are Carbon, Nitrogen, Oxygen, Fluorine, Neon. Carbon alone is only element 6, not the full run.

Litmus paper is a common pH indicator made from a mixture of dyes derived from lichens. For a more precise measurement, full-range pH indicator paper or a pH meter is used.

Deposition is the phase change in which a gas transforms directly into a solid without passing through the liquid phase. Condensation is the process by which water vapor cools and turns back into liquid water droplets.

Elements 11-15 are Sodium, Magnesium, Aluminum, Silicon, Phosphorus. Neighboring element blocks shift the atomic-number range.

Elements 16-20 are Sulfur, Chlorine, Argon, Potassium, Calcium. Neighboring blocks give the wrong atomic numbers.

When an acid and a base react together, they neutralize each other and produce a salt and water. The classic example: hydrochloric acid + sodium hydroxide → sodium chloride + water.

Common states are solid, liquid, gas, and plasma. Atoms, molecules, and compounds are particle/substance categories, not states.

Atoms contain protons, neutrons, and electrons. Molecules and compounds are atom combinations, not subatomic particles.

An element is a pure substance made entirely of one kind of atom. A compound is a substance formed when two or more elements are chemically bonded together in a fixed ratio.

A compound is a substance formed when two or more elements are chemically bonded together in a fixed ratio. Compounds have properties completely different from the elements they contain.

A mixture is different from a compound in a critical way: in a mixture, substances are physically combined but not chemically bonded. Trail mix, salad, ocean water, and air are all mixtures.

The atomic number of an element equals the number of protons in the nucleus of its atoms. This is the defining number for each element — changing the proton count changes the element entirely.

A physical change alters the form, size, shape, or state of a substance without changing its chemical identity. Cutting, bending, dissolving salt in water, and melting ice are physical changes.

pH 7 is neutral; lower values are acidic and higher values basic. pH 0 is strongly acidic, not neutral.

Condensation happens when water vapor cools and becomes liquid droplets.

Conductivity is the ability to let electric current or heat flow through a material.

Relative density compares how dense an object is against a fluid such as water, helping explain floating and sinking.

Melting is the change from a solid to a liquid, such as ice becoming water.

Condensation is the change from gas to liquid, such as water vapor forming droplets on a cold surface.

Density compares how much matter is packed into a given volume. Less dense objects can float in denser liquids.

The human body has 11 major organ systems, each with a specialized role but all working together. While each system has its own function, none works in isolation.

Integumentary system is skin, hair, and nails. Muscular system moves the body, not the protective covering.

Skeletal system supports, protects organs, and makes blood cells. Muscular system moves the body; it is not the framework.

The muscular system enables all movement of the body and helps maintain posture. The skeletal system provides the body's structural framework, protects vital organs, and produces blood cells in the bone marrow.

The nervous system controls and coordinates all body functions using electrical signals called nerve impulses. The endocrine system regulates growth, metabolism, mood, reproduction.

The endocrine system regulates growth, metabolism, mood, reproduction. The nervous system controls and coordinates all body functions using electrical signals called nerve impulses.

Cardiovascular system pumps blood carrying oxygen and nutrients. Respiratory system exchanges gases; it does not circulate blood.

The lymphatic system serves two key functions: it drains excess fluid from body tissues back into the bloodstream. The cardiovascular system transports blood; the respiratory system exchanges oxygen and carbon dioxide.

The respiratory system brings oxygen into the body and removes carbon dioxide — the gas exchange that sustains life. The cardiovascular system transports blood; the respiratory system exchanges oxygen and carbon dioxide.

The digestive system breaks down food into nutrients small enough to be absorbed into the bloodstream. The respiratory system brings oxygen into the body and removes carbon dioxide — the gas exchange that sustains life.

The urinary system filters blood and removes liquid waste from the body as urine. The digestive system breaks down food into nutrients small enough to be absorbed into the bloodstream.

The reproductive system is responsible for producing offspring and continuing the species. The endocrine system regulates growth, metabolism, mood, reproduction.

The scientific method is the systematic process scientists use to investigate questions about the natural world. The steps are: Observation — noticing something interesting; Question — asking why or how; Hypothesis — forming a testable prediction; Experiment .

A hypothesis is a testable prediction. It is NOT a random guess; it must be based on something you already know or have observed.

The independent variable is the one variable that a scientist deliberately changes in an experiment. The dependent variable is what you measure or observe in an experiment — it is the 'output' or 'effect.

The dependent variable is what you measure or observe in an experiment — it is the 'output' or 'effect. The independent variable is the one variable that a scientist deliberately changes in an experiment.

The control group is the group in an experiment that does NOT receive the experimental treatment. Without a control group, you cannot know if the results are due to your experimental variable or some.

In science, a theory is not a guess. A hypothesis is a testable prediction.

A scientific law is a statement — often expressed as a mathematical equation — that describes a consistent, observed pattern in nature. In science, a theory is not a guess.

Homeostasis is the body's ability to maintain a stable internal environment despite changes in the outside world. Metabolism is chemical activity; homeostasis is maintaining stable internal conditions.

The skin is the largest organ of the human body, covering the entire external surface. The liver is the largest internal organ and performs over 500 functions.

DNA stores the main hereditary code. RNA helps use that code, so it is not the primary instruction archive.

A cell carries out life functions; an atom is a unit of matter, not life. A neutral atom has balanced charge; an ion has gained or lost electrons.

A ruler measures length or distance.

A thermometer measures temperature.

A gram is a metric unit of mass.

A graduated cylinder measures liquid volume accurately.