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|Maxwell's demon |
An imaginary creature who is able to sort fast-moving molecules from slow-moving molecules without adding any energy to the system. Such a creature could separate a gas into two containers, one containing hot gas and the other cold gas, bringing about a general decrease in entropy and violating the second law of thermodynamics; however, such a creature is impossible, since the act of observing the molecules to detect their speed must add energy to the system.