* Temperature is a measure of the average kinetic energy of molecules. Molecules in a hotter object are moving faster and have more energy than molecules in a colder object.

* When two objects of different temperatures are in contact, the faster-moving molecules in the hotter object collide with the slower-moving molecules in the colder object. This collision transfers energy, causing the slower molecules to speed up and the faster molecules to slow down.

* This energy transfer continues until both objects reach the same temperature. At this point, the average kinetic energy of the molecules in both objects is equal.

Therefore, heat always flows in the direction that equalizes the temperature difference between two objects.