A balloon's worst fear is a sharp object, so usually when you puncture a balloon, it pops in your face. Not with this science trick! To puncture a fully blown up latex balloon without popping it, you'll need a pointed metal or wooden skewer and some plain old dish soap. That's it.
Steve Spangler explains how it's done in this video, but it's as simple as coating the balloon with a little bit of dishwashing liquid, then slowly push your sharpened skewer into one of the least stretched out portions of the balloon, usually near the rim where you tied it, using surgical precision. Then, move it slowly through the inside of the balloon and out the top.