Rubber bands can store enormous amounts of potential energy, at least as far as toy cars are concerned. When wrapped around a rigid axle attached to wheels, a rubber band can create enough torque to propel your toy car forward at impressive speeds.
Building a self-propelled car that can make the most of this stored energy is a relatively simple matter, requiring no more than some simple tools and materials and a little free time.
Cut out a 5-inch by 6-inch square of cardboard, and then cut a 2-inch wide and 1.5-inch notch into one of the 5-inch sides. Throw away the notch.
Thread the skewer across the notch by sticking it through the cardboard's corrugated edges. The skewer should cross the open space left by cutting out the notch.
Put a CD onto each protruding end of the wooden skewer, outside the cut out notch. Cement them to the skewer with poster putty.
Stick a thumbtack through the skewer, centred in the middle of the notch.
Stick the other two thumbtacks next to one another, spaced an inch apart and positioned at the un-notched 5-inch side of the cardboard.
Wrap one end of the rubber band around the thumbtacks in the cardboard and the other end around the skewer-bound thumbtack.
Twist the skewer in a circular motion toward the cardboard-stuck thumbtacks, which will wind up the rubber band. After two complete revolutions, set the car down on a flat surface, and release it. The stored energy of the rubber band will rotate the tires and propel the car forward.