Pop pop boats function with the help of a heat engine consisting of a small boiler, and this boiler attaches to a tube. This tube is known as the exhaust pipe. Some boats have one exhaust pipe, while others have two. When the boiler is heated, water from the boiler becomes steam and pushes out water through the exhaust tube, which propels the boat forward. The bubble of steam then condenses. On condensing, a vacuum creates and water is drawn back through the exhaust tube. The cycle then repeats, when this cooled water brought back into the boiler is again heated.
People often mistake the working of pop-pop boats. They assume that since water is sucked in and thrown out of this exhaust tube, the boat instead of propelling forward will move back and forth. However, one must understand that water propels out only in one direction. However, during the second-half water draws back into the exhaust pipe equally from all directions. This asymmetry helps propel the boat to move forward.
Boats with two exhaust pipes have the system of passing water through both tubes in the first phase, and sucking in water from both tubes in the second phase. Therefore, water does not go in through one exhaust and out through the other exhaust.
Blowing out a candle applies this same principle. Extinguishing a candle by blowing air over it is very easy. This is because the air expelled moves only in a certain direction. However, when it comes to distinguishing a candle by sucking in air, then it becomes all the more difficult.