Rub your hands together rapidly. You'll quickly notice that you feel warmth from the friction of the two hands in contact. You have converted mechanical energy is a device that can convert thermal energy into mechanical energy. It operates continuously and only into thermal energy quite easily. The reverse process is more difficult. A heat engine allows heat and work to pass across its boundaries. Typically, a heat engine will use thermal energy to do work and then exhaust the heat that cannot be used to do work. You may be wondering how a heat engine relates to thermodynamics.