Homemade toys and craft projects can be a great means of teaching children about scientific principals--in the case of a self-propelled boat, propulsion, tension and what causes motion. A simple self-propelled boat, made out of balsa wood, cardboard or polystyrene, uses the elasticity of a rubber band to create motion.
Lay the balsa wood on a cutting surface and draw the shape of a boat hull on one end. On the other end draw a square, 3 inches wide and 3 inches deep. Leave one inch on either side of the square, with the end of the balsa wood comprising one of the four sides of the square.
Cut along the pencil lines with the knife, so that one end is a pointy boat shape and the other end has a square cut out of it.
Cut the piece of balsa wood that came out of the square-cut piece on the boat, so that it is a 1x2-inch rectangle. This will be the paddle
String the rubber band over the end of the boat, so that it passes over the centre of the square cut out portion, and around the sides of the boat.
Thread the paddle through the centre of the rubber band, and twist it around repeatedly so that the rubber band is wound tightly between the boat frame and the paddle.
Hold the tension of the rubber band and place the boat in a small body of water--a swimming pool or bathtub will do fine. Release the hold on the rubber band and the boat will self-propel as the rubber band unwinds and moves the paddle around in the process.