Heat is the random, microscopic movement of molecules: when the tea is hotter, the movement of the molecules is more agitated. Why does it cool down? Boltzmann hazarded a splendid hypothesis: because the number of possible states of the molecules in hot tea and cold air is smaller than the number in cool tea and slightly warmer air. The combined state evolves from a situation where there are fewer possible states, to a situation where there are more possible states. The tea can’t warm itself up, because information cannot increase by itself.
