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. The name comes from the noise made by some versions of the boats.
A pop pop boat is powered by a very simple heat engine. This engine consists of a small boiler, which is connected to an exhaust tube. When heat is applied to the boiler, water in the boiler evaporates, producing steam. The expanding steam is suddenly pushed out of the boiler, making a 'pop' sound, and pushes some of the water out of the exhaust tube, propelling the boat forward. The boiler is now dry, and can therefore not generate any more steam. The momentum of the column of water in the exhaust tube keeps it moving outward, so that the pressure inside the boiler drops below atmospheric pressure. In the case of a diaphragm type engine, the boiler also bulges inward at this point, also making popping sound. The pressure outside the boiler now forces water back into the boiler. This water then boils and the cycle repeats. The popping noise is more pronounced when a diaphragm-type boiler is used: coil-type boilers are much quieter.
Any air in the boiler can act as a spring and support the oscillation of the water, but if too much air enters the boiler, the oscillation will stop because all the water has been displaced, and no steam can be generated. Water contains some dissolved air, which can build up in the engine during operation. Therefore, engines must "burp" out air periodically in order to run for a long time.
In pop pop boats with two exhaust tubes, the water is expelled from both tubes during the first phase of the cycle, and drawn in from both tubes during the second phase of the cycle. The water does not circulate in through one tube and out through the other. The internal-combustion analog of the pop pop boat engine is the valveless pulse jet.