Refrigeration (from the Latin frigus, frost) is generally the process of cooling of a body by the transfer of a portion of its heat away from it. Cooling of something hot is often done by means of material at ambient temperature, for example the fan cooling of computer equipment. Where temperatures below that of any available natural cooling agents are required, refrigerators are used to produce the required cooling effect by taking in heat at low temperatures and rejecting it at temperatures somewhat above that of the natural cooling agent, which is generally water or air. The function of a refrigerating machine, therefore, is to take in heat at a low temperature and reject it at a higher one, using external energy to drive the process. A refrigerator is effectively a heat pump, a heat engine running in reverse. It is also possible to use eutectic salts.