From Nature:
James Clerk Maxwell came up with his thought experiment in 1867. In it, a demon guards a door between two rooms filled with gas. Using its sprightly demonic powers, the creature could open the door when he spotted a particularly fast-moving molecule coming his way. The molecule could then pass into a room, which would become progressively hotter. Likewise, the demon could allow particularly slow-moving molecules to pass out of the warmer room and into the cooler one. By doing so, he creates a growing temperature difference, and therefore, potential energy in the system, without having expended any energy to do it (assuming our magic demon doesn’t eat).
In the real world, researchers have made little devices that might be used to make a demon-like machine. One of these is a ring-shaped molecule, which is slotted onto a tiny molecular axle. The ring can move along the axle between two different sites, A and B. If left to its own devices, the normal, random movement of molecules will shunt the ring back and forth. When there are many devices, at any given time, half of them should have a ring at one site, and half at the other.
More here.
