1. Take the milk caps and poke holes in them the same size as your sticks.
2. Put the sticks through the straw and put milk caps on each end of the sticks which creates a wheel and axle set. Make two of these to form a set of four wheels for your car.
3. Cut out a rectangular piece of cardboard, making it whatever size you want your car to be.
4. Tape the wheelsets to the cardboard piece.
5. Put a balloon over a straw and tape the balloon onto the straw.
6. Tape the balloon and straw onto the frame of the car.
7. Blow into the balloon, set it on the ground and watch it go. It will go in the opposite direction that the air is flowing in since for every force, there is an equal and opposite force.
8. Decorate it however you see fit.
Here is the video to go along with this posting: http://www.youtube.com/watch?v=-fuCoA0iWZI.