A pop-pop boat is a toy with a very simple steam engine without moving parts, typically powered by a candle or vegetable oil burner
Commercial pop pop boats have usually been made out of tinplate. The hull of the boat may be made out of any material that floats. The boat's engine consists of a boiler and one or more exhaust tubes. While a single exhaust tube may be used, two exhaust tubes are much more commonly used. This is because the boiler and the exhaust tubes have to be filled with water, and using two tubes allows water to be injected into one tube while air inside the engine escapes through the other tube. It is more difficult to remove the air and completely fill single exhaust tube types. The boiler and exhaust tubes are usually made out of metal, with tin or copper being common. The physics of the operation of the pop pop boat is very similar to that of the "Feynman sprinkler," a submerged sprinkler which is seen to turn weakly or not at all as water is sucked in through it. In both cases, the reaction force on the solid device caused by the sucking in of the fluid is balanced by the fluid impinging on the inside of the device.