Page "Evaporation" Paragraph 2

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid.

With sufficient heat, the liquid would turn into vapor quickly ( see boiling point ).

When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide.

Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to ' escape ' ( evaporate ).