Boiling is a special phenomenon in Physics, more accurately called vaporization (vapourization in Canada), when the water molecules start to leave the water। This forms a vapor that is usually (but not always) visible to the naked eye. Most people interpret this phenomenon as: When the temperature of a solution is raised, it starts to boil. This is simple, but not necessarily true. In fact, the way vaporization works is: When the temperature of a solution is raised, its pressure also increases, when it's partial pressure (or vaporization pressure) is equal to the surrounding atmospheric pressure, it vaporizes.
Figure 1: A pot of water that was brought just to boiling (vaporization)
This can be pretty confusing, but basically it states that there are two ways to boil water: Raise the temperature, or drop the atmospheric pressure!
WARNING: NERDY PART!
The actual physics behind this goes as follows: As you start to raise the temperature in the solution, the molecules become more active. As these molecules become active the pressure increases, when the pressure inside the solution gets high enough they will actually start to burst out of the solution. This 'bursting' action is interpretted as vaporization.
So with this new found knowledge here's some fun. First you will need some sort of a sealed vial, as you can see in image 2. Then you fill the vial half-way with water. You will need some way to evacuate some of the air above where the water is, a small valve can be handy. Then you need to cool down the air above the water in the vial - what this will do is drop the pressure of the air above the water, this is the water's atmospheric pressure. If you cool it enough (dropping the atmospheric pressure) the water will begin to boil. That's right - a cold boil, good luck making tea with this water!
Figure 2: Sealed vials.
Some crazy facts:
The pressure in high altitude places is lower than low altitude places, so when you boil water it boils at too low a temperature to make food!
The pressure in space is insanely low (not zero though, that's impossible) so if you are in space and you open your mouth your saliva will boil.
