Hold the bottom bulb in the palm of your hand and wrap your fingers around it. The heat of your hand increases the temperature of the gas in the bottom bulb. The increase in temperature creates an increase in air pressure; the gas expands. This increased pressure pushes the liquid in the bottom bulb up the tube to the top bulb. When all of the liquid has transferred from the bottom bulb, air bubbles are forced up the tube from the bottom and this causes the liquid in the top to look like it is boiling.