The little tin boat has a tiny kettle with an exhaust system. First water is injected into the "kettle" through one of the exhaust system's pipes. When water overflows out of the second pipe, the kettle is full. Then a little candle is lit and inserted underneath the boat's kettle. The small amount of water quickly turns to steam. Before the steam actually finds its way out of the tailpipes, the steam molecules push against the closed end of the kettle and drive the boat forward. Once the pressure drops inside the kettle, one would suspect that the boat would stop. But it doesn't because atmospheric pressure pushes water from outside the boat back into the hot kettle, which vaporizes the incoming water and raises the pressure again. The little putt-putt sound heard from the boat is evidence for that cycle. The boat moves without interruption until the candle's wax runs out or until one removes the burning candle.