Say it like this

The pull that everything with weight has on everything else. Bigger things pull harder.

For the grown-up

Gravity is the mutual attraction between masses. It is what holds you to the floor, keeps the moon circling the earth, and assembles gas clouds into stars. It is also extraordinarily weak, which is why it takes something planet-sized to make it obvious.

The easy mistake

Careful here. Gravity is not something the earth does to you. You pull on the earth too, by exactly as much. The earth is simply far too big to notice.

Words next to this one

Orbit Going round and round something because it keeps pulling you in and you keep missing it. Star An enormous ball of burning gas, so far away it looks like a dot. Our sun is one. Galaxy An enormous group of stars travelling together. Ours has a couple of hundred billion in it. Universe Everything there is, including you. Most of it is empty space.