[fowlerrr]

If I can remember my high school physics, basically it's because of the low pressure.

Pressure x Volume = Temperature, so as the pressure decreases, so does the temperature.

But Wikipedia answers it better than I can: http://en.wikipedia.org/wiki/Boiling_point

[jjs]

You're looking for the Ideal Gas Law (http://en.wikipedia.org/wiki/Ideal_gas_law), which is derived from Boyle's Law (relation between gas pressure and volume), and Charles's Law (expansion of gas when heated).

The related concept here is that liquids remain liquid when the vapor pressure of the substance is less than the atmospheric pressure.

So, by the above laws, you can make water boil by (a) heating the water to increase the vapor pressure, or (b) reducing the atmospheric pressure to less than the existing vapor pressure of the water, or (c) some combination of (a) and (b).