Materials: electric soldering iron, four bottle caps with the same size, a plastic bottle, a balloon, a straw, a rubber band and two sweater needles. Glue making process: after the temperature of the electric soldering iron reaches a certain level, burn four small holes in the bottle cap, so that the sweater needles can rotate freely in the holes. Note: the radius of the bottle cap must be larger than the distance from the small holes to the bottom of the bottle. Then burn a hole in the center of the bottle cap with an electric soldering iron, and then fix the four bottle caps on both sides of the bottle with wool needles to form four small wheels. Then the balloon is put on the straw, tied tightly with a rubber band, and then a groove which is not much different from the straw is made in the back of the bottle with an electric soldering iron. Pay attention to the oblique downward branding. The straw is being fixed with glue (strong glue such as 502). Note that because the bottle cap is too smooth, the edge of the bottle cap is covered with a rubber band to reduce its friction. When blowing, the balloon will become larger and tilt to both sides, so insert two wool needles on the bottle body to form a cross, so that the balloon can be fixed. The production of the power car is completed. Usage: Blow air into the balloon through the straw, then release the gas in it, and the car moves under the action of recoil force.