Maxwell's demon

/ măks′wĕlz′,-wəlz /

1. 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.