Maxwell's Demon

James Clerk Maxwell created a thought experiment that suggests the possibility of violating the second law of thermodynamics. In this hypothetical, a demon opens and closes a door between two chambers that lets a single molecule pass from one chamber to the other. If the demon lets fast moving molecules pass only in one direction and slow moving molecules only in the other. One chamber will heat up while the other cools down (Using 'free' Brownian motion to decrease entropy). While the practicality of any such real mechanism on any scale and no matter how abstract, is easily dismissed, the thought of it lead to many investigations of the relationships between thermodynamics and information theory.