How does Santa get Down the Chimney? Science!

Christmas is attendant with many mysteries -- like what the hell does it actually have to do with Jesus Christ? (um, nothing) -- but one important question is why does Santa use the chimney and also how does he use it (well, that's two questions) and why is it important that he's so fat (okay, three questions), but they all have one answer: air pressure.

This does not answer the question of how Santa gets back up the chimney, which I believe is not "by lighting a fire under his butt (that would just flood the house with smoke ... and kill Santa) but by being tethered to magic flying reindeer.