Making a self propelled car is a common physic project assigned to elementary ag ed children. The easiest way to accomplish this is to use a tissue box, two penc ils, rubber bands, and thread spools Materials: Buy our Balloon and Rubber Band Racecar Kit to get everything you need to build a rubber band car and a balloon car. Or, find the following materials: Car body: sturdy cardboard, foam board, or balsa wood Straws Axles: wooden dowel that fits inside the straws Wheels: use plastic bottle caps, film canister caps, foam board, toy wheels such as K'nex, wooden wheels from a craft store, etc. Rubber bands 2 small cup hooks What to do : Cut a six-inch length of balsa wood to be the car body, or chassis. Cut a 1x1-in ch notch out of one end. Your rear axle will be accessible through this notch. Cut two lengths of straw the same width as the chassis and glue or tape them acr oss the chassis near each end. Trim away the center of the rear piece where it s tretches across the notch in the chassis. Make sure the straws are lined up stra ight, or else the car won't roll straight. (The picture to the right shows how i t should look underneath the car.) Cut the dowel into two pieces, each one inch longer than the width of the chassi s. Thread the dowels through the straws to create your axles. Slide a cup hook o nto the rear axle so that it fits tightly in the center of the notch in the chas sis. The screw will stick up to catch the rubber band. (If you don't have the ri ght-size cup hook, you can make a catch by wrapping an unfolded paperclip around the axle and securing it with hot glue.) Attach your wheels to the axles. The wheels need to be a tight fit on the axle; if they aren't, you may want to use hot glue to hold them in place. Screw a small cup hook into the chassis just behind the front axle. Choose a large rubber band to power the car. Loop one end on the hook at the fro nt of the car, and loop the other end over the catch on the rear axle. Your car is ready to roll! Turn the rear axle several times to wind the rubber b and around it, set the car on a smooth hard surface, and let go! (Note: if your wheels are smooth, you might need more traction for the car to operate properly. Wrap rubber bands or loops cut from a small balloon around the rear wheels to a dd traction.) What's happening: The more you twisted the rubber band around the axle, the more potential energy you built up. When you let go, the rubber band snapped back to its original form , spinning the axle in the process. The potential energy in the stretched band w as converted into kinetic energy propelling the car forward! There are many ways you could change your car design to make it go faster or far ther. Experiment with different types of wheels. Will the car go farther if you use bigger wheels, or wheels with more or less friction? What if you use bigger wheels in the back and smaller in the front? Or a 3-wheeled design? Try building a car with CDs for wheels. Does the weight of your car affect how it travels? T ry adding a load like coins or washers to the car and see how it changes the dis tance or speed. What happens if you make your chassis longer? If you give your c ar a ramp to start on, how much further will it travel?