Maxwell's Demon is a thought experiment proposed by physicist James Clerk Maxwell in 1867. It imagines a tiny creature that can sort particles in a gas into hot and cold groups without expending energy. By allowing only fast-moving particles to pass through one door and slow-moving ones through another, the demon seemingly creates a temperature difference, challenging the second law of thermodynamics. This scenario raises questions about the nature of entropy and information. While the demon appears to violate thermodynamic principles, later analyses show that the act of measurement and sorting requires energy, thus preserving the second law.