My daughter got a Magic Science kit for Christmas. In this picture, she's seeing what happens when you mix baking soda (a base) and citric acid in cold water, and then in hot water. In cold water, you get a fizz, but with hot water, you get a fizz-splosion! The chemical reaction happened faster in the hot water because the water molecules were moving faster. The experiment illustrates why we refrigerate food - to slow down chemical reactions.