Here's why:

* Exothermic reactions release heat into the surroundings, causing the temperature to increase.

* Endothermic reactions absorb heat from the surroundings, causing the temperature to decrease.

Chemical heat packs work by using a chemical reaction that releases heat, thus warming up the pack. The most common reaction used is the oxidation of iron, which produces heat and rust.

Let me know if you have any more questions!