Do you know the saying “what goes up, must come down”? There’s a lot of truth to that. No matter how hard you hit that baseball or how high you get on the swings, you’re not going to make it into space (without a spaceship, of course). This is because of something called gravity (GRAV-it-ee). Gravity is the force that keeps you (and all your toys) from floating into space.
The Earth’s gravity is a force that works kind of like a magnet. When you jump in the air, you come back down because gravity is pulling you towards the center of the Earth. Gravity does a lot more than just keep your feet on the ground. The strong pull of planets has created whole solar systems and galaxies. The Earth's gravity pulls in the Moon, which orbits (or circles) around it. Objects that orbit planets are called satellites (SAT-uh-lights). Some other planets have one or more moons of their own. The largest planet in our solar system, Jupiter, has 63 known moons! The Sun also has a gravitational (grav-uh-TAY-shun-ull) pull. It pulls all the planets in our solar system around it. Just like the Moon circles the Earth, the Earth circles the Sun.
This force is something that all objects have—even you! The reason you don’t have tiny objects stuck to you is because you’re not big enough to have a strong enough pull. Even really big things like whales aren’t large enough to have a gravitational pull. Only really, really big things like stars, planets, and moons have it.
The Moon is big enough to have its own pull. Its gravity tugs on the Earth's oceans. That's why we have ocean tides. But the Moon's gravity isn't as strong as the Earth’s. That’s why the astronauts who visited the Moon were able to jump really high. If those same astronauts went to a bigger planet, like Jupiter, the gravity would be a lot stronger. There, they would feel much heavier, and they wouldn't be able to jump much at all. People in spaceships are not near anything with a big gravitational force, so they can float in the air inside the spaceship.