Perpetual motion
A perpetual motion machine of the second kind spontaneously converts thermal energy into mechanical work. In itself, this does not violate the law of the conservation of energy, because there is the same amount of mechanical work than there was of thermal energy. Such machines violate the Second law of thermodynamics. Real heat engines, such as those found in cars, also convert heat energy to work. Such engines have two reservoirs: the heat is transferred from the hot one to the cold one. Some of the heat is converted into mechanical work, the rest is used to heat the cold reservoir. Perpetual motion machines only have one reservoir for the heat. For this reason, they can convert all the heat into mechanical work.