The source of energy used in driving many forms of machinery is the oil or coal consumed - usually in the boiler-room of the power plant. When this oil or coal is burned it gives off heat. The heat converts water into steam, and the expansion of the steam drives the engine. The steam that issues from a steam locomotive or from an open pipe of a power plant, like the steam that is given off from a kettle on the stove, is a watery (aqueous) vapor and is always found when water is heated. Steam resembles common air and other gases in many of its properties. It differs from gases in that it does not retain permanently its gaseous condition. For this reason it is not called a gas but an aqueous vapor. The white cloud of vapor noticed when steam is liberated is due to water particles in suspension in the air.

The chief property or characteristic of steam is its elasticity which makes it capable of enormous expansion.