Hold the bottom bulb in thepalm of your hand and wrap yourfingers around it. Theheat of your hand increasesthe temperature of the gas in thebottom bulb. The increase in temperature creates an increasein air pressure; the gasexpands. This increasedpressure pushes the liquid inthe bottom bulb up the tubeto the top bulb. When all ofthe liquid has transferred fromthe bottom bulb, air bubblesare forced up the tube from thebottom and this causes theliquid in the top to look like it isboiling.