The answer would be:
Evaporation
If you'd like to know more about this, read on:
Evaporation is the process where a substance in a liquid state is changed to a gaseous state. This occurs when there is an increase in temperature or pressure. When water gains a lot of heat, it changes to gas in the form of vapor. The amount of vapor depends greatly on the amount of water present. For example, in a desert, you might think that there is a lot of evaporation going on, but the answer is no. This is because there is not much water present.