Heat never flows from cold to hot on its own — the universe has a preferred direction

The second law of thermodynamics says the total entropy, or disorder, of an isolated system can only stay the same or increase over time. It's why a cracked egg never uncracks itself and a hot drink cools rather than heating up further on its own. Rudolf Clausius formalised the law in 1850 and coined the term 'entropy' itself in 1865; unlike almost every other law of physics, this one gives time a direction.

— Rudolf Clausius, On the Moving Force of Heat — 1850; term 'entropy' coined 1865