Evaporation

Evaporation is the process by which molecules in a liquid state spontaneously become gaseous. It is the opposite of condensation. Generally, evaporation can be seen by the gradual disappearance of a liquid, when exposed to a significant volume of gas.
On average, the molecules do not have enough energy to escape from the liquid, or else the liquid would turn into vapor quickly. 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 that one of the molecules ends up with enough energy to be considered past the boiling point of the liquid. If this happens near the surface of the liquid it may actually fly off into the gas and thus "evaporate".
Liquids that do not appear to evaporate visibly at a given temperature in a given gas have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the "escape velocity" - the heat energy - necessary to turn into vapor. However, these liquids are evaporating, it's just that the process is much slower and thus significantly less visible.
Evaporation is an essential part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration are collectively termed evapotranspiration.