Magic Hand Boiler
A hand boiler is a toy made out of hand-blown glass and filled with a special liquid called ethyl alcohol. The boiler consists of a larger lower bulb and smaller upper bulb, connected together by a twisting glass tube. This liquid inside the boiler is special because it appears to boil at a low temperature. And all it needs to do so is the heat of your hand. As you probably know, water boils at a temperature of about 100 degrees Celsius. With a hand boiler, the temperature of your hand is enough to cause the liquid to bubble and appear to boil.
Check out a video of a hand boiler: When holding the bottom of the boiler with your hand, the liquid inside becomes exposed to the heat of your hand and it begins to evaporate. Its pressure increases and the liquid expands. As a result it shoots up the glass tube and settles in the top bulb, since it cannot go any further. Next, the vapor bubbles (created by the evaporation process) travel up through the glass tube from the bottom bulb and into the liquid in the top bulb. These vapor bubbles force themselves through the liquid and cause it to bubble, giving the appearance of boiling. This process continues as long as the heat of your hand is applied to the bottom bulb.
The liquid inside hand boilers is typically given different colors for visual appeal, and also to better show the liquid as it bubbles and moves through the boiler.
To force the liquid in the top bulb to go back down into the bottom bulb, hold the top bulb with your hand and the heat of your hand will increase the liquid pressure in the top bulb and force the liquid back down into the bottom bulb.
If you have cold hands the hand boiler will not work as well since the heat from your hand is not high enough to cause the liquid to evaporate and boil. You can try it out on a cold winter day, after you come in from outside. See what happens!