I'm not sure if I got the name right, but anyways... Suppose we have a nanoscopically small, but very clever device. (and with modern technology, this sort of thing is becoming almost practical...) It is basically just two compartments, with a hole in between. Contained in the compartments, we have an energetic gas. The hole is controlled by a very clever valve mechanism, that lets particles through in one way only. We let the device run, and hey presto, by simple random motion, all of the gas is gathered in one of the compartments. At the push of a button, the hole can be used to drive a turbine from the diffusion of the gas, thus generating energy - and decreasing entropy. Obviously, such a device would break the laws of thermodynamics. So, why exactly would it not work?