What's a planet?

Planets are big round things that go around the sun. Planets are made of different things, like rocks, metal, water, and gas. Earth is a planet. Some other planets are smaller than Earth, and some are much bigger. Earth has a moon. Some of the other planets have more than one moon, and some don’t have any moons at all. The moon is big and round, but it isn’t a planet because it goes around the Earth. Asteroids and comets go around the sun like planets, but they are not big or round. Pluto is round but it is not a planet.