This heat pad is made from a supersaturated solution of sodium acetate trihydrate. Above +58 degrees Celsius sodium acetate trihydrate is always a liquid; below about -10 degrees Celsius it is a solid. Normally, the salt would crystallize as it cools below +58 degrees Celsius, except under special conditions such as with the heat pads. Thus, it can easily exist at room temperature as a supercooled liquid. The trigger button is the mechanism for producing the seed crystals that initiate the phase change. The phase change from liquid to solid releases latent heat, making these heat pads very useful for easily contained calorimetry experiments and a host of hands-on activities.