A Pop Pop Boat is a very simple steam engine, with no moving parts.

The heat source can be a small candle or a sold fuel tablet. Inside the boat there is a small boiler containing water. This leads to two small metal tubes at the back of the boat, which are under the water. When the water in the boiler is heated to boiling point, it turns into steam which is pushed out of the exhaust tubes, and it also pushes out some water from the exhaust tubes. This propels the boat forwards. The pressure inside the boiler is now less than atmospheric pressure, and so more water is drawn into the boiler, and the process repeats.

Pop Pop Boat date from the 19th Century. This particular one was made by the late Ron Fuller, who was a friend of Tim's.