This program illustrates a famous thought experiment which raised important issues about the nature of entropy. In 1871, James Clerk Maxwell imagined a situation where a very small and nimble being could, by opening and closing a valve between two gas-filled chambers, allow only faster molecules to go through one way and only slower molecules to go through the other. By doing this, the being (later called a demon) could raise the temperature of one chamber and lower the temperature of the other, without the expenditure of work. This was a violation of the Second Law of Thermodynamics, and he asserted that this could not actually occur. The implications of this puzzle have continued to be central to thermodynamics, entropy, and information theory up to the present time.
For detailed instructions on using the program, take a look at the info window from the Maxwells Demon model.
Click on one of the pictures to see a quicktime movie of the model: