Refrigerators, air conditioners, and heat pumps all utilize the temperature-pressure-volume relationship. Have students follow the cycle shown in the following video and point out where expansion/evaporation causes a drop in temperature resulting in heat moving from the surroundings into the coils, i.e., refrigeration/air conditioning. Likewise point out where compression/condensation causes increase in temperature resulting in heat moving from the coils into the surroundings. Heat pumps are the same; they differ only in which end of the system you are sitting at. Note that there is no overall cooling/heating as such. The appliance only facilitates the flow of heat energy from one location to another.