When water is heated, it undergoes a sudden transition from liquid to vapor at a specific temperature. Why does this happen? Two water molecules tend to attract each other when they are close enough to each other, but there is a limit to how close they can get because each water molecule has a non-zero volume (roughly speaking). This article uses these two simple inputs to deduce the existence of a liquid/vapor phase transition, which is characterized by a sudden change in density when the temperature reaches a special value.
