Thermodynamics is the science of heat and energy and their relation. It grew out of the study of gases and out of the engineering of steam engines, which generate heat by burning something, then transform that heat's energy into useful mechanical energy. The science treats heat and energy as equivalent, distinguishing between usable heat/energy versus relatively-evenly-distributed heat/energy that cannot be transformed into mechanical energy in isolation, and includes laws regarding the limits of transformations of such heat/energy. The principles have been extended to chemical energy, e.g., heat released or absorbed by chemical reactions. Statistical mechanics models the underlying basis.