Probably because her mom told her to. There are actually two things that are affected when you add salt to water. First, the boiling point increases (remember boiling-point elevation from the chapter “Macroscopic Properties: The World We See”?). Second, the heat capacity, or how much energy it takes to raise the temperature, decreases. While you might think this would change how fast your pot of water boils, the amount of salt you would have to add to see a significant change is pretty large. The real reason you add salt to water for cooking? Flavor. Salt tastes good.