Liquids can evaporate to become gases. The gases formed from liquids are called vapors. If a liquid is injected into a rigid, evacuated container (a container from which all matter has been removed, including gaseous matter), the liquid at first evaporates. Evaporation continues until a state of equilibrium is reached between the gas (vapor) and liquid. The pressure of the gas in the container at this point is called the (equilibrium) vapor pressure. The process is imagined to be like this:

The molecules that evaporate must overcome attractive forces (break bonds) in order to enter the gas state. The energy to break these bonds comes from the energy of motion. The kinetic energy is 'converted' into potential energy (the potential for reforming the broken bond). Cooling takes place. As the kinetic energy is used in this fashion, the amount available decreases, the average decreases, and the temperature falls. In a sealed container, this process is limited. Once the equilibrium vapor pressure is reached, the extent of net evaporation reaches zero.

The amount of energy involved can be substantial. Direct evaporative cooling can lower energy costs substantially. Limitations are humidity (doesn't work in very high humidity) and water costs (places where it works well, like deserts, often place a premium on water use).