What is the second law of thermodynamics?

The second law says a description of a system can only drift toward the settled description, never away from it. Measure the gap as relative entropy, the extra yes-or-no questions your odds cost compared with the settled odds. Send two different messages down the same noisy channel and they come out harder to tell apart, and nothing done to both afterwards separates them again. Repair is a channel of that kind and leaves the settled state alone, so the gap only shrinks. Plain entropy is a different number and can fall. Settling a disagreement takes two possibilities to one, and that bit leaves as heat. A three-state machine that only ever steps clockwise obeys the law with a current that never dies. Started on one state, its gap falls from 1.585 bits to 0.585, then 0.085, then 0.024.

Also in Chapter 23: Why Does Writing Something Down Cost Heat?.