It’s also hot just when you stretch it. Then it cools to room temperature. When you relax it, it cools below room temperature.
It’s similar to how refrigerators work, compressing gas, cooling the hot compressed gas to ambient temperature, then allowing the gas to expand quickly and cool down.
It’s a cool thing to show kids, but I think difficult to explain, even to adults. Look up “adiabatic”- that word alone is pretty scary.