Why does air rise at the equator and sinks near the poles?

The first two answers are correct; hot air rises. In the atmosphere, it's a little more complicated than this because of the Earth's rotation. Air rises at the equator and sinks at the poles, but it also sinks near latitudes 30 degrees north and south. That's why all the world's big deserts are near these latitudes; sinking air doesn't release any rain. And it rises in temperate latitudes in the middle of the depressions which give us rain in places like Europe, America and NZ. So each hemisphere has three convection cells, not just one.