These toy Pop-pop engines, or water-impulse engines, date back a hundred years or so. Their origins are obscure, which adds to their charm and mystery. The pop-pop engine is simply a tube of coiled copper that has both its ends in the water. The coil is heated and the boat in which it is installed moves forward.
How this works, in scientific terms, is still open to conjecture and it has been the source of some controversy. What appears to happen is that a little water in the tube flashes into steam, forcing the rest of the water out the back, making the boat go forward (see one of Newton's various laws). The vacuum thus created within the tube then draws in more water and the cycle repeats.
In practice there are many variables, as follows: the length and diameter of the tube, the heat source, the weight of the boat, and whether or not the flame is shielded. Any and all of these will affect performance.
Pop-pops have always been the stepchildren of the steamboat family. They can truly be called steamboats, but they bear little resemblance to a proper Steamboat. They have no moving parts, after all. In the early days of pop-poppery, the German manufacturers often built a thin, flexible metal diaphragm into the water/steam circuit. This diaphragm was dished slightly and, when the water flashed into steam, the thing would spring out, making a noise like a metal cricket that you snap by pressing it with your thumb. That's how the boat got its common name. (The Germans call them toc-toc boats for the same reason.) Similar inexpensive boats are available today from India.
Want to make one yourself? Take a look at these DIY pop pop boat instructions!