When a physical or chemical change happens, energy moves between the stuff that is changing (the system) and everything around it (the surroundings). Energy is never created or destroyed, only transferred. An exothermic change releases heat into the surroundings, so the surroundings warm up. An endothermic change absorbs heat from the surroundings, so the surroundings cool down. A thermometer in the mixture tells you which one happened, and a foam cup with a lid helps keep the heat from leaking away while you measure.