The narrow copper coil of the flash boiler holds a miniscule volume of water, so the heat of the alcohol burner almost instantaneously flashes it to steam. The expanding steam forces water out of one of the two tailpipes. Because the tube is completely full of water and sealed, fresh cold water is simultaneously drawn in through the other tailpipe. This new water then flashes to steam, expands, and bursts out through the exhaust, drawing more fresh cold water in its wake, and so on. The boat putters forward—gaining speed and momentum—until the burner goes out. Since the coil is symmetrical, either tailpipe can function as the intake or exhaust on a given voyage; that’s for the vagaries of chance and thermodynamics to determine.

You might wonder how it is that the boat moves forward at all, since the intake is drawing in the same volume of fluid as the exhaust expels, at the same speed. At first glance, it seems like the forces should balance and the boat go nowhere (or, possibly, spin about its tail). The trick is that the exhaust, focused by the straight tailpipe, is coming out in a jet stream, while the intake is drawing in water from the broad, hemispherical region surrounding its opening. The forceful, directional exhaust stream overpowers the unfocused intake stream and thus propels the boat forward. Let this be a lesson to you.