I was thinking to myself, why does cold air go to the bottom of the room and the hot air to the top? At first the answer looked simple, the cold air is "denser" than the hot air. But if you think about it, at the molecular level the only difference between hot air and cold air is that the molecules of the hot air are further apart an are more energetic (please correct me if I'm wrong). If the fact that they are further apart (less dense) has to do with it, It seems like gravity pulls down on the air molecules makeing the bottom of the room have less that the top - this obviously isn't true cause if it were, this would also happen in a room starting with one temp. , the bottom would get cold and the top hot due to gravity. So it sees as if the reason that the hot air goes up is because of the fact that it iis more energetic - but what does that have to do with it?