It turns out Scotch tape curls up when exposed to humidity, so it can be used as a water-grabbing claw. This could be used to monitor water quality, according to the scientists at Purdue who developed them. The cellulose-acetate side, the slick clear film you rub with your finger, is water-absorbent. The adhesive side you stick on wrapping paper is water-repellent. So when one side absorbs water, it expands while the other side remains the same. The result is a curled-up piece of tape.