Pop-pop boats come in a wide variety of shapes and sizes, but they are mostly cheap toys in the classical sense. Most available today are modern reproductions of toys made 50-100 years ago. Very few of the originals survive. This example is called a "Steam Boat Number 1". I think it's made by Welby, but I'm not 100% sure about that.
Many purists would probably claim that this wonderful little toy doesn't belong in this collection, because it has no cylinders, pistons or crankshafts. Indeed, it has no moving parts of any sort. However, not only is it steam powered, it is very definitely a toy! Exactly how it works is a mystery to most folk, including myself. The usual explanation sounds convincing enough until I try to work out the details, then I'm not so sure I understand. It's a bit like the steam injector; it definitely works but the theory behind it is bound up in the subtleties of thermodynamics.
The "works" consist of a small chamber from which lead two parallel pipes terminating at the stern end under the waterline. And that's it! You fill the chamber with water, place a source of heat under the chamber and, from there, the usual explanation goes as follows: The water in the chamber boils forcing the water in the two pipes out through the back end providing a small impulse to the boat. The cold water then re-entering the pipes, condenses the remaining steam which sucks in more cold water to refill the chamber. The whole process then repeats indefinitely (until the heat source runs out). This all happens quite rapidly and the characteristic sound produced gives this type of boat it's name. This sounds feasible enough at first, but, why doesn't the water being sucked in at the end of the cycle cancel out the impulse of the water being blown out? Why two pipes? How can the whole process repeat so rapidly (several times a second) when the theory implies that the whole chamber full of water is boiled and re-condensed completely at each cycle?
The heat source can be almost any source of flame small enough to fit under the chamber. With the boat when purchased was a supply of tiny candles and also a small spoon shaped device with a wick, the recommended fuel being cooking oil! Once warmed up (after a couple of minutes) it pops along happily at a few inches per second. There is no rudder on this example. This is inconvenient because there is no means of control (other than sinking it!) and it can go on for hours using one of the candles.
Construction is entirely pressed "tin-plate". Painting is minimal covering only half of the inside of the hull. The only embellishment is a circular logo pressed into the top stating "Steam Boat Number 1 India"