Step 1: Have the kids measure the width of axles of the car. This measurement determines how wide to make the air-powered car body. Lego has wheels and axles available that work well for this experiment.

Step 2: From the measurement in step 1, have the kids first trace the shape of the car onto a flat piece of Styrofoam. Make the width of the car slightly less than the axles so the wheels extend slightly from the body and do not rub the side of the car. If you are using building blocks you can skip this step.

Step 3: Cut out the car body. Adults should assist kids with this step or have the body sections pre-cut to save time. If you want to allow the kids time, have them decorate their cars with paint, markers, paper, or stickers. Using building blocks will speed up this step.

Step 4: Attach the axles and wheels to the car body using strong tape. Make sure the car wheels are aligned and spin well. How well the wheels spin will determine how much energy from the balloon is transferred to speed of the air car. Lego has some great car wheels with plastic axles that snap together. Also, water bottle caps, plastic push up sticks, and other things you might throw out make nice light weight wheels. Use your imagination!

Step 5: Supply a balloon size and straw (or tubing) size that fit well together. The balloon opening should fit tightly around the straw without crushing the straw. Attach the balloon to the straw or tubing using duct tape.

Step 6: Attach the balloon and straw to the wheels and car base. Blow up the balloon. Pinch off the air temporarily. Line up the car in the direction of the race track and release the air. If there are multiple children who are performing the car experiment, they can all race their cars to test the design. Let the kids make adjustments on their cars and allow them time to race again.