Consider this completely hypothetical situation. You hurt your knee playing basketball and need a cold pack quick. Since the gym is right next door to the chemistry lab and it just so happens that there are no ice packs left in the freezer, and the ice machine quit working in the lunchroom, you ask your best friend to go ask the Chemistry teacher to whip you up a cold pack from a chemical reaction. Which type of reaction would you need the teacher to perform in order to make a homemade cold pack for your knee? Explain why.