A heat engine is a mechanism that converts heat or thermal energy to mechanical work in thermodynamics. Heat engines such as gasoline and diesel engines, jet engines, and steam turbines all function by using some of the heat transfer from a source. The term "heat engine" can be used to describe devices that use any form of heat other than nuclear energy or electricity. The word "engine" here does not have its usual meaning of a motorized vehicle but rather any mechanical device that uses energy stored in a compressed state to produce continuous motion. Thus, an engine can be as simple as a weight on a string but must also include a reservoir where it stores energy before it is released by the mechanism which produces power. Engines are most commonly seen in vehicles but they also appear in water pumps, lawnmowers, and many other tools used around the house and garden. They are also used in some small scale industrial processes. So, a heat engine is simply a machine that uses the properties of heat to do work. By far, the most common type of heat engine is your car's engine. It converts electrical energy into kinetic energy (motion) which allows your car to move forward. Your car's engine uses combustion (burning) to convert energy from the gasoline you pump into movement! Another type of heat engine is your home heating system.