Ok, so I have complete faith in the tried and proven laws of thermodynamics, including the part of the second law which states that it is impossible to create a heat engine working in cycle that takes in a certain amount of heat energy, and converts all the heat energy it takes in, into Mechanical work. But I am counfused, with regards to how a steam engine would not violate that law. In particular, this is what I am confused about: Couldn't you build a steam engine, where the steam is ran through just enough rotors that the temperature gained from heating the steam is in effect all lost to turning the rotors in the turbine, and thereby is returend to the boiler in the state that it left it in, thereby making no need for a cold sink for the heat to be dumped into? I know that my reasoning is flawed somewhere, but I'm not sure where . . . . .