A heat engine is a device that repeatedly converts thermal energy into kinetic energy. It needs the difference of temperature and a working substance with a high rate of expansion to do useful work for us. It can be done in two ways: utilizing the change of volume of the working substance that hot and cold reservoirs make alternately expand or compress, and utilizing convection that the change of density resulting from the change of volume causes. A heat engine is not confined to an artificial one. The Earth also does useful work for living systems as a heat engine. The heat engine whose hot reservoir is solar radiation heat maintains the circulation of air and water, while the heat engine whose hot reservoir is geothermal heat maintains the circulation of mantle and mineral nutrition.