This would be evaporation. Evaporation occurs when a liquid changes to a gas. Also, bonus facts:
When a gas changes to a liquid, this is called condensation. When a liquid changes to a solid, this is called freezing. When a solid changes to a liquid, this is called melting. When dry ice turns into carbon dioxide gas, this is called sublimation. Most other solids could probably do this, but dry ice is the most common example.