A steam locomotive is a locomotive whose primary power source is a steam engine. The most common form of steam locomotive also contains a boiler to generate the steam used by the engine. The water in the boiler is heated by burning combustible material – usually coal, wood, or oil – to produce steam. The steam moves reciprocating pistons which are connected to the locomotive's main wheels, known as the "driving wheels". Both fuel and water supplies are carried with the locomotive, either on the locomotive itself, in bunkers and tanks, (this arrangement is known as a "tank locomotive") or pulled behind the locomotive, in tenders, (this arrangement is known as a "tender locomotive").