The internal combustion engine is an engine in which the combustion of fuel an an oxidizer (typically air) occurs in a confined space called a combustion chamber. This exothermic reaction creates gases at high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement of solid parts of the engine, by acting on pistons, rotors, or even by pressing on and moving the entire engine itself.