Figure 2.

Maxwell’s demon: a being who knows the velocity of every particle in the box and can select their passage, using a opening in the wall that divides it, which could separate those with high energy from those with low energy without performing work, thus violating the second law of thermodynamics. The demon has to forget the past states of the system but, according to Landauer’s principle, this process generates heat (at least $kTlog2$ J per bit erased) and entropy.