Maxwell's demon

Maxwell's demon is a thought experiment from the 1800's created by scientist James Clerk Maxwell. In the experiment, there is a tiny creature, called a demon, that sits between two chambers of gas. The gas on one side is hotter than the other side, but normally the two gases would mix together and become the same temperature. However, the demon is able to open and close a door between the two chambers. When the demon sees a fast moving molecule on the hot side, he opens the door and lets it escape into the cold side, thus making the hot side even hotter and the cold side even colder. In this way, the demon is able to separate the hot molecules from the cold molecules, and eventually the two sides will have different temperatures.