Want to do an experiment, demonstrate some of the principles presented here? Something as simple as a rubber band, when stretched and relaxed, will act as a simple heat pump.

1. Acquire a rubber band, preferably a large one (this is your physics instrument).
2. Place the large, flat part on your lips (this is your temperature sensor).
3. Stretch the rubber band while holding it to your lips. What do you notice about its temperature?
4. Let the rubber band relax back to its original shape. What change do you notice?

If you have conducted this experiment correctly, you will notice the band's temperature rise when you stretch it, and you will notice the temperature drop when you relax the band.

The rubber band changes temperature for a different reason ¹ than the traditional heat pump, but the effect is easy to sense and the test equipment doesn't require a research grant.

(1) It turns out that the rubber band heats up and cools off due to the changing order of the molecules in the rubber band and a quantity called entropy, but I digress.