I looked up the youtube movie of this experiment. It's pretty amazing.
The idea is to take a flask partly filled with water and heat it up to boiling. Then put a stopper in it. It now pretty much stops boiling. Icing the part that doesn't have water in it causes more boiling. How?
Boiling depends on both temperature and pressure. When water boils, it means that it any water vapor bubble in it grows as water molecules enter it from the liquid more than it shrinks as water molecules leave it for the liquid. Those bubbles are just what you see in the boiling. Water molecules leave the liquid much faster when it's hot. They also leave the the vapor faster when it's under more pressure. So lowering the pressure makes it easier to boil. Water up on a mountain at lower pressure dose indeed boil at lower pressure than it does farther down.
Icing the part of the flask with air in it slightly lowers the pressure, because the air molecules rattle around less when they're colder. That would cause a little bit of water to boil, until the pressure builds back up. There's a much more important effect. The flask mostly has water vapor, not air, up in that part. Icing it cools it enough so that there the vapor condenses back to liquid. That lowers the pressure a lot, causing more boiling. So with the main part of the flask very hot and the top part almost as cold as ice, water keeps boiling up from the hot part and condensing at the cold part.
The drip of cold water will gradually cool the hot water, so the process won't go on for ever.
(published on 04/12/2012)