A heat engine, or more simply, an engine, is a device that extracts energy from its environment in the form of heat and does useful work. At the heart of every engine is a working substance. In a steam engine, the working substance is water, in both its vapor and its liquid form. In an automobile engine, the working substance is a gasoline–air mixture. If an engine is to do work on a sustained basis, the working substance must operate in a cycle; that is, the working substance must pass through a closed series of thermodynamic processes, called strokes, returning again and again to each state in its cycle.