Temperature, as you know, measures how hot or cold a body is with respect to a standard object. To discuss temperature changes, two basic concepts are important: thermal contact and thermal equilibrium. Two objects are in thermal contact if they can affect each other's temperature. Thermal equilibrium exists when two objects in thermal contact no longer affect each other's temperature. For example, if a carton of milk from the refrigerator is set on the kitchen countertop, the two objects are in thermal contact. After several hours, their temperatures are the same, and they are then in thermal equilibrium.