A pull-back race car relies on a spring attached to the back axle, which gets wound tightly when the car's pulled back and returns to its normal shape when released, propelling the car forward. Make one of these toy cars at home for a fairly easy, very fun afternoon science project.
Building the Car
Fold the piece of paper in half. Orient the page with the long edge facing you and fold the right edge over to the left side. Open the fold and spread glue inside the paper, then fold it and leave it to dry for two or three minutes.
Fold the paper again, so the crease meets the opposite edge and glue the page together again.
Glue or tape the pencils to the top side of the paper, along the edges of the car. This will make up the car chassis. The pencils should be parallel to each other, and less than 1/4 inch from the edge of the paper.
Measure and mark a 1-1/2-inch square notch in the center of what will become the back of your car.
Cut the square you marked out of the car.
Cut a plastic drinking straw into two segments, each as long as the car is wide.
Glue or tape these segments to the underside of the car chassis. Both should be about 3/4 inches from the absolute front and back of the car. These straw segments will hold the axle in place.
Cut the middle inch out of the rear straw, so there are two axle holders on either side of the notch, and they're lined up exactly. Take care not to move the straw when cutting.
Clip the binder clip to the front of the car, but leave the arms in the open position.
Remove both the top and bottom arm on the binder clip.
Loop one of the binder clip arms through the elastic band, then reattach it to the top of the binder clip. Flip the arm to the closed position.
Cut the dowels into two axles, each should be wide enough to fit through the car, accommodate the wheels, and have less that 1/8-inch tolerance between the wheels and the chassis. Too loose, and the car won't roll straight, too tight and it will roll more slowly.
Attach one wheel to each of the dowels, and slide the dowels into the straws on the base of the car. Place the rear wheel on its axle through the two straw segments, and the front wheel assembly through the front axle.
Flip the car over and attach the other wheels.
Glue, staple and tape the elastic band onto the exposed part of the rear axle. Make the connection as firm as possible, then leave any glue to dry for at least 20 minutes.
Using the Car
Bring the vehicle to a flat, open area.
Pull the car backwards along the floor or surface. The rubber band will wrap around the rear axle as you do so. If you want to lift it, hold the back wheels so the elastic doesn't pull back into place--this will allow you to pull back two or three times. Just make sure you don't break the elastic.
Release the car and rear wheels to send the car racing forward.