Maxwell's Demon
Maxwell's Demon is a thought experiment designed to challenge whether the second law of thermodynamics would always hold. This demon has the ability to open a slot or a divider that divides two gas chambers, quickly enough so that only the fastest molecules of the gas pass the divider in a certain direction. This would theoretically create a difference in temperature of the gas in each chamber, which challenges the second law that entropy should always decrease in an isolated system. In this case, the difference in temperature raises the potential energy of the system, which shouldn't be the case if there is no tranfser of heat.