Now attach the balloon to the car to make it move.

Put the long straw into the neck of the balloon. Push it in about 2cm and then secure it with an elastic band.

Make a hole in one end of the box and push the straw and balloon through. Set the straw at an angle so that the end of it is pointing towards the floor. Hold the straw in place by wrapping an elastic band around it on the outside of the box.