A well insulated container is placed on top of a magnetic stirrer. The container is filled with hot water and then a cylinder of (cold) copper is placed in the water with a lid sealing the container. There is a thermometer in the water and a second thermometer close to the center of the copper cylinder.

We observe that the temperature of the water drops whereas the temperature of the copper goes up. This process stops when the temperatures of water and of copper have become equal. (Close inspection shows deviations from this simple picture due to the interaction with the container and the environment.)