Let's pretend that gravity stopped. It is only make-believe, but it shows what gravity does for us all day. Gravity is the invisible pull that makes things fall down and keeps you on the ground. With no gravity, nothing would hold you there. When you throw something up, gravity pulls it back down. Without gravity, nothing would pull the ball back.

Gravity holds down the air we breathe, so Earth keeps its blanket of air. Without gravity, Earth would not keep that blanket. Gravity also moves the sea. The Moon's gravity pulls on the seas, and that makes the ocean tides go up and down. Without the Moon's pull, there would be no such tides.

Your weight is how hard gravity pulls on you, and the pull comes from all of Earth's mass. Your mass, the amount of stuff you are made of, stays the same even when your weight changes. On the Moon you would weigh about one-sixth of what you weigh on Earth, because the Moon pulls less. With no pull at all, you would have no weight.

Everything that has mass has gravity, and things with more mass pull harder. Gravity gets weaker the farther apart two things are. The Sun is a giant ball of hydrogen gas, and gravity holds all of that gas together. It would take almost 1,000 Jupiters to fill up the Sun. Without gravity, nothing would hold that gas together.

Gravity keeps the Moon going around Earth, and Earth going around the Sun. The Moon takes about 27 Earth days to circle Earth. Earth takes about 365.25 days to go once around the Sun. Earth's average speed is about 29.8 kilometers per second, or about 107,000 kilometers per hour. The Sun's gravity keeps Earth at a comfortable distance for its light and warmth.

Everything in orbit is falling around its planet. It moves sideways so fast that it keeps missing the ground. The International Space Station is in low Earth orbit, and it circles the planet once in about 90 minutes. Without gravity, there would be nothing pulling the station around Earth.

You may have heard that astronauts float because there is no gravity in space. In fact, gravity does not stop in space. Astronauts float because they are in free fall, and everything inside the station is falling together. Anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it. On a roller coaster, free fall gives you a brief feeling of weightlessness at the top of a drop.

Words to know

• gravity: invisible pull
• mass: amount of stuff
• orbit: path around something
• tides: rising falling seas
• weightless: without weight