Heat always flows from hot objects to cold ones, and this direction gives time its forward arrow.
Pour warm milk into a cup of cold tea: they mix and reach the same temperature. That never happens in reverse on its own. Rovelli shows that this simple rule about heat, not some mysterious law, explains why the past differs from the future and why time seems to move forward.
Why it matters The idea explains why every machine loses some energy as heat, no matter how well it is built.