Cut both skewers (starting from the pointy end) to be a bit longer than the straw halves.If you have really sharp scissors you can cut right through the skewers. Otherwise, cut through as much as you can and then snap the skewers off.
Make a hole in the center of the bottle caps using a small screwdriver.To do this, place the cap top up on a cutting board and push the screwdriver down into the center of the cap. Then pick up the cap and wiggle the screw driver all the way through until the hole is just large enough for the skewer to fit snugly into.
Add a cap to the pointy end of the skewer, top first, and push it to the other side.It should be a snug fit, but you can add some hot glue inside the cap to keep it in place if necessary.
Then add a straw half onto the skewer.
Place a second bottle cap on the skewer, opposite to how the first cap was added, so both tops of the caps are pointing out.Repeat with the other caps, skewer, and straw half to make a second axle with wheels.
Cut off the pointy end of the skewer so no one gets hurt. You can also trim the other end of the skewer if it sticks out too much. (We left between 1/4 and 1/2 inch outside the caps.)
Step 3: Connect the wheels to the 'car body'
Tape the straws to one side of a plastic water bottle, a couple inches in from each end. Make sure that the straws are perpendicular to the bottle so that the wheels roll in the right direction.You can use patterned washi tape to help decorate your car, or simply use regular tape to attach the straws.
Step 4: Add balloon power to the car
Place the bottle with the wheels down and put the second paper straw into the bottle's mouth, on an angle so it hits the top of the car/bottle. Hold the straw so that an inch is outside the mouth of the bottle. Then mark an inch below where the straw hits the inside of the bottle. (You can either make an actual mark with a marker, or just keep your finger on the spot like we did.)
Remove the straw and use a craft knife (or scissors) to cut an 'X' shape where you marked on top of the bottle.
Slide a balloon onto the second straw and secure it to the end with a small elastic.
Push the straw in through the 'X' cut on the bottle, until the end of the straw comes out the mouth of the bottle. (There should be about 1 inch coming out the mouth of the bottle and 1 inch coming out the top of the bottle on the balloon end. )
Now put your mouth on the straw end that's sticking out of the bottle mouth. Blow into the straw, which will in turn blow up the balloon.The elastic should keep the balloon in place and secure on the straw as it blows up, but if it leaks air you may need to adjust the elastic on the balloon or glue the balloon to the straw.Now put your finger on the end of the straw to hold in the air, and then let it go to watch the balloon car drive away!