 Maxwell’s demon showed that entropy could be decreased: The second law of thermodynamics implies that heat won’t pass from a cold body to a hot one. In classical thermodynamics this law is quite strict; but in Maxwell’s kinetic theory of heat there is a probability, though extremely small, of such an event happening. Some thought this a reductio ad absurdum of Maxwell’s theory. To show how it is possible to violate the second law, Maxwell imagined a tiny creature who controls a door between two chambers. Fast molecules from the cold box are let into the hot box, and slow molecules from the hot are allowed into the cold. Thus, there will be an increase in the average speed in the hot box and a decrease in the average speed of molecules in the cold. Since, on Maxwell’s theory, heat is just the average speed of the molecules, there has been a flow of heat from a cold body to a hot one.