In terms of energy, a person doing physical work can be compared to a combustion engine. The engine converts chemical energy of gasoline or oil into mechanical performance but with certain losses. Similarly, in the human body, a large part of the energy it receives is 'wasted' by being converted into heat and only a small proportion becomes useful mechanical energy. In both engine and human the term efficiency denotes the ratio between the externally measurable useful effort and the energy consumption that was necessary to produce it.