Well, the sky isn't really... the sky. I mean, it is the sky, but space is just that - space. Empty space. The stars are dense balls of gas held together through gravity and pressure. And the sun isn't actually moving - it's the earth that is. The earth, like a star, is a dense ball of mass held together through pressure and gravity, and it rotates around the sun and also turns in a circle itself. So when the sun goes down, it's really just the earth rotating enough so that that side of the earth is not facing the sun.

_{[ Was that 101 enough? He wasn't sure... ]}