The handboiler is made of glass. A tube connecting the top chamber to the bottom chamber extends nearly to the bottom. There is a liquid that is deep enough to submerge the lower end of the tube. Above the liquid both in the base, in the rest of the tube and in the top chamber is the vapor from that liquid. Those two pockets of vapor are therefore not connected. Applying warmth from hand to the bottom chamber creates a pressure difference from the bottom to the top. The liquid is then forced slowly up the tube towards the top chamber. The boiling action is the result of the fluid level exhausting itself in the bottom chamber and allowing the vapor to escape up the tube.