Source: Martha Stewart Kids
Talk about blowing away the competition! This air-powered carton car needs just a huff and a puff to get its little wheels rolling.
Use a clean, half-pint milk carton. Loosely tape a balloon to the top so that air can get through. For the wheels, slide two thin dowels through two straws (the dowels should be thinner and slightly longer than the straws). A parent should use an awl to puncture a small hole in four plastic jar or bottle lids (four of the same size or two sets of different sizes). Slide the ends of dowels into the holes of lids (the holes should be small, so that the dowels fit snugly into them). Tape the straws to the bottom of the carton. If the car seems unbalanced, tape a washer to carton's bottom or side. Rev up the "engine" by inflating the balloon, and then release so that the car can get going.