A well insulated container having two equally sized compartments s placed on top of two magnetic stirrers (one for each compartment). The first compartment is filled with cold water and the second one with hot water (equal amounts). The container is sealed with an insulated lid. There is a thermometer in each of the bodies of water.

We observe that the temperature of the hot water drops whereas the temperature of the cold water climbs. This process stops when the temperatures of the two bodies of water have become equal (with a value almost precisely halfway between the two initial temperatures).

(Close inspection shows deviations from this simple picture due to the interaction with the environment: the equilibrated temperatures of both bodies of water drop very slowly.)