Learn more physics!
How does hot air expand , and how does cold air contract ?
What it means for something to be hot is that all the little parts are
rattling around intensely. For a gas, that means that the fast-moving
molecules will bang into things often and hard. In other words, the gas
will exert more pressure on its surroundings. Often, what happens is
that the gas pushes things (say balloon walls or neighboring gas) out.
That leaves more room for the hot gas. The pressure drops, partly
because the expanded gas just doesn't have as high a density of
molecules bumping into things.
If the gas is cooled, its pressure drops, and neighboring things
push in until the density is high enough so that the pressure is big
enough to equal the pressure of the surroundings.
(republished on 07/27/06)
Follow-up on this answer.