This Geyser Pen works very much like a real geyser. The water below a geyser is heated by thermal activity. A geyser erupts when its water reaches a temperature high enough for it to overcome the pressure at that depth, and then it shoots the water up to the surface. When you hold the Geyser Pen with your hand, you warm the liquid at the bottom of the pen. Just like a geyser, the liquid is then forced up the tube creating bubbles in the upper chamber.