Like cicadas, these toys appear on the market every so often, occasionally as a craze, then disappear again for another 17 years. So I reckon I was wise to take the opportunity to acquire one whilst I had it.

Patented in 1892 by Desiré T Piot, nobody seems to know how the pop-pop motor really works, though theories abound, the most reputable candidates based on the Stirling Cycle. The engine consists of nothing more than two metal tubes sprouting from a simple flattish boiler like a teaspoon, the top of which is a flexible diaphragm of brass shim. The boiler is heated underneath by a camping stove fuel block or a piece of candle, during which the diaphragm vibrates, driving the tiny boat forward like a pulse-jet.

Back in 2004 New Scientist was kind enough to publish an explanation of mine for how it worked, following which Jeff Bindon of the University of KwaZulu-Natal, South Africa kindly sent me a complimentary pop-pop motor he had built. In place of the brass-shim diaphragm the boiler had a piece of kevlar cut from an overhead projector foil, allowing you to see inside as it worked. Although Jeff diplomatically forbore to draw attention to the fact, it confirmed to my complete satisfaction that my published explanation was totally cock-eyed.

© 2010, Clark Nida.