How to Make a Balloon Powered Car

Build a simple balloon-powered car using everyday materials for a fun physics experiment with your kids.

1. Gather Your Materials. You'll need a plastic water bottle (16-20 oz works best), four bottle caps of the same size for wheels, two wooden skewers or straight straws for axles, a balloon, tape, and a pair of scissors. Optional items include markers for decorating and clay or playdough to add weight if needed. Make sure all materials are clean and the bottle caps are from the same type of container so your wheels will be even.
2. Prepare the Car Body. Remove the label from your plastic bottle and clean it thoroughly. This will be your car's body. If you want to decorate it, now is the perfect time to let your kids use markers to create designs, racing stripes, or their favorite colors. Make sure any decorations are dry before moving to the next step.
3. Create the Axles and Attach Wheels. Carefully poke holes in the bottle near the bottom and top, making sure they're aligned so your car will roll straight. The holes should be just big enough for your skewers or straws to fit through snugly. Thread the skewers through the holes to create axles. Next, poke small holes in the center of each bottle cap and push them onto the ends of the skewers. Make sure the wheels spin freely but don't fall off. Trim any excess skewer length, leaving about half an inch beyond each wheel.
4. Attach the Balloon. Stretch the balloon opening over the mouth of the bottle. Make sure it's secure by wrapping tape around the neck of the balloon where it meets the bottle opening. The balloon should be firmly attached so no air can escape except through the balloon opening. Test this by gently pulling on the balloon to ensure it won't come loose.
5. Test and Launch Your Car. Blow up the balloon through the bottle opening, then pinch the balloon neck closed to keep the air inside. Place your car on a smooth, flat surface like a hallway or driveway. Release the balloon neck and watch your car zoom forward as the air rushes out. The escaping air pushes backward, which propels the car forward, demonstrating action and reaction forces.
6. Experiment and Improve. Try different modifications to see how they affect your car's performance. Add weight by taping coins to the bottle, change the balloon size, or adjust the wheel alignment. Test your car on different surfaces and measure how far it travels each time. Keep track of which changes make the car go farther or faster, and discuss with your kids why certain modifications work better than others.