Here’s something you probably didn’t know. I thought it was interesting when I first learned it. When you heat water to a great enough temperature it boils. That is, the liquid turns to gas and escapes. You might think, then, that heat is the cause of boiling. It’s not.
To understand what makes water boil, it helps to understand what makes it a liquid. What makes the water in, say, a lake a liquid is atmospheric pressure. The weight of the atmosphere pushes with enough force that molecules of water are compressed together. The natural kinetic energy of these molecules that would normally drive them apart is overcome by the weight of the atmosphere. In other words, they are squeezed together by atmospheric pressure much like you would squeeze a piece of foam larger than your hand (analagous to water vapour) into a small more condensed ball of foam that fits inside your closed fist (analagous to water).
Now, some water molecules (from bouncing around against other molecules in the the liquid mix) end up gaining enough energy that they break free of the liquid and are absorbed by the atmosphere above. We call this process “evaporation” and over time it reduces the amount of liquid water to zero unless it is replenished via another method (e.g. rain, river inflow, etc.). If the atmosphere is already full of water molecules (i.e. it’s humid), evaporation happens more slowly than if the atmosphere is empty of water molecules. This is because when the air is humid, molecules that exit the water into the air are more or less balanced off by the molecules in the air getting trapped back in the water.
When you heat water, this gives the molecules in the water more energy. More energy gives the molecules the ability to break out of the constraints of liquid form more readily. That is, the pressure of the atmosphere is overcome by the additional energy of the water molecules granted them by heating. Thus liquid water becomes gaseous water more quickly than by the ambient temperature process of evaporation.
Here’s the neat part. As I’ve described above, whether water molecules exist in liquid form or in gaseous form is the result of the interplay of two factors: a) the kinetic energy of the molecules, and b) the (atmospheric) pressure under which it exists. It follows, then, that you can cause water to boil not only by giving the molecules more energy — as by heating — but you can also do it by reducing the pressure that surrounds it so that less energy is required to escape liquid form. On the top of Mount Everest, for example, where atmospheric pressure is a quarter what it is at sea level, water boils at a measely 71°Celsius — a much lower temperature than the 100°Celsius you would expect in normal experience. And in space, where there is no atmospheric pressure (nor, it’s worth noting, heat) at all, water will boil almost instantly — at least, whatever doesn’t freeze. Ergo, all other things being equal, the boiling of water is primarily a function of the pressure under which it exists. Heat is not required to boil water at all, but is rather something that can facilitate the process by granting water molecules more energy to overcome the pressure that surrounds it.