1. The problem statement, all variables and given/known data We have three metal cubes, the same material, same size. Two cubes have temerature 300K, third 100K. On what maximal temperature can we heat the on of the cubes? 3. The attempt at a solution If we want to get higher temperature than 300K we need to use the gradient of temperatures two cubes to get some work, which we can after use to heating the third cube. I tried to place ideal heat machine, which work on base of carnot cycle between the two cubes (one 300K other 100K) - but I just have two temperatures and on wiki is written, that to work from carnot cycle I need entrpy too. So I don't know how to continue. Thanks for any help.