This represents the simplest steam engine. The boat is made from a plastic water bottle cut in half lengthwise, a quarter inch diameter copper tube bent to have a 360 degree loop, and plumbers putty to keep the boat from leaking. The tube is completely filled with water and a candle placed under the upper part of the tubing as shown in the pictures. When the tubing becomes hot enough to boil the water, the engine shoots water alternatively out one exhaust and then the other, propelling the boat forward. Why does water coming out propel the boat forward, while water coming in does nothing? See Cartesian Diver with a Twist.

Putt Putt Steam Boat