My question involves heat engines. I understand that a heat engine typically uses energy provided in the form of heat to do work. According to the 2nd law of thermodynamics, however, not all heat energy can be converted into work energy, meaning that heat engines are not perfect. At least some of the energy must be passed on to heat a low-temperature (cold) energy sink. "A heat engine with 100% efficiency is thermodynamically impossible." Why is this so? Why can't do we need the cold reservoir? Why can't all the heat energy become work energy? Thanks!