Hands Down, an Edmund Classic! -- Hand Boilers are looped and twisted glass sculptures containing a liquid that will "boil" as heat is transferred from your bare hand. Simply hold the large bulb end in the palm of your hand and within minutes, your body heat will cause the liquid to rise. - Once out of your hands, it takes a few minutes for the liquid to cool down and return to the large bulb. Pretty fascinating science toy and an excellent demonstration of energy transfer. Comes in three different colors: blue, green and red - let us pick one for you.