Add lukewarm water to two plastic cups. (Do not use insulating Styrofoam cups.) Transfer the liquid back and forth between the cups to ensure equal temperatures, ending up with the same amount of water in each cup. Add several tablespoons of table salt to one cup and stir. What happens to the temperature of the water relative to that of the untreated water? (Hold the cups up to your cheeks to tell.) Is this an exothermic or endothermic process? What’s going on at the molecular level?