Water is introduced by placing one of the rear pipes under a tap. It is well made and works effectively. The candle provided, or, for more dramatic effect, a piece of solid fuel, is lit beneath the tiny boiler which rapidly expands and contracts drawing in and expelling water to propel the boat and make the characterisitc 'pop-pop' sound.
13 cm POP POP boats now appear as part of the Schools National curriculum for GCSE Science !!