You can make the body of the boat with a recycled, cut-up milk or juice carton, which is easy to cut, water-proof and paintable (although mine are not painted here).
Gabriel from California sent me this wonderful picture of his daughter and her science fair project about pop pop boats. Here is an interesting article that describes pulsating heat pipes (PHP) which incorporate the same principles that make pop pop boats work. Steam, which is the gaseous version of water, can be used to power trains, lights in buildings, and your very own model boat! Take thick gauge wire (or an untwisted wire coat hanger) and wrap the front and back cigar tube or steel pipe so the tube is firmly and securely wrapped with equal lengths of wire sticking out from each side.

Mount the cigar tube to your Styrofoam board by inserting the wire ends into the Styrofoam. Pouring hot water in the tube and then heating it with tea candles will cause the water to vaporize and turn to steam. Where A is the cross-sectional area (or the size of the hole) and v is the velocity that fluid comes out. The steam, which is a gas, expands and escapes through the hole at a velocity proportional to the size of the hole.
In either case the engine is the same, made from a cut-up aluminum soft drink can and drinking straws.

You can see other people's projects (and add yours if you send a picture) in the GALLERY OF POP POP BOATS. Because very action has an equal and opposite reaction, the boat will move forward as the steam pushes out of the end of the tube.

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