is an 1867 of James Clerk Maxwell suggesting how the second law of thermodynamics might hypothetically be violated. The description was contained in his Theory of Heat in 1871. As individual gas molecules approach a small hole, his demon quickly opens and shuts it to allow only fast molecules to passed from chamber A into chamber B, and only the slower particles to pass back from chamber B. This causes the the second chamber to warm and the original chamber to cool, even though the chambers are