Thread a plastic drinking straw onto a length of string. Tie each end to a chair, cabinet handle, or other sturdy anchor point; the two anchor points should be about the same height. Move the straw to one end of the string. Blow up a balloon and pinch it shut (don't tie it). Use masking or scotch tape to attach the balloon to the straw, as shown. Let the balloon go and watch it zip down the string!
What's happening: The air escaping the balloon provides the "fuel" for this rocket. When you blow up a balloon, you're increasing air pressure inside. When you let the balloon go, the air escapes through the nozzle, creating thrust that propels the balloon in the opposite direction -- in this case, down the string track.
Originally published in the April 2012 issue of FamilyFun magazine.