Have you ever pulled a muscle or just been sore after a long day of work or exercise? Hot showers are great, but maybe you've used the more convenient heat packs. Heat packs, which you can buy at grocery or drug stores to soothe aching muscles, use exothermic reactions to produce a low level of heat that lasts for an extended period of time. Exothermic reactions change chemical energy into heat energy. In this chemistry science fair project, you will use heat packs to study the heat-generating reaction of oxygen with the iron within them.