This helps kids see the effects of catalysts on chem reactions. The catalyst in this case being heat.
Demo this with NO explanation and have kids decide what's behind it and why its so important to chem. Obtain a cyalume light stick. These are the chemically activated usually greenish light sources used in camping emergencies. The cheapest place to get them is a camping supply store. Science equipment places will mark it up a million %. Have a beaker of cold water and a beaker of hot water. In a darkened room, snap the light stick on. Kids will go, "Oooo". Now, after their eyes are adjusted to the dim surroundings, place half of the stick in the cold water. They should notice the light dims drastically. Now place it in the hot water - the light brightens drastically. Why? Simple demo that heat is the great catalyst for most chem reactions. The chemical reaction that is producing light is hindered by the lack of heat and helped by the addition of heat. Ta Da!