Get a carton and cut off the top. Cut it in half long ways. Cut a small hole in the middle of one of the short sides. This is going to be where the balloon will go. Put the balloon through the hole. Do not blow it up yet. Make holes on each side of the carton for the axles and stick straws through these holes. Put these close to the bottom. Attach the spools to the straws and put pins through the straws to keep the spools on. Blow up the balloon but don’t tie it. This is when you let the balloon go so the car will go.