Glue manufacturers must find a balance between two things when making the sticky substance — how strong it holds and how easily it can be pulled apart. That's because boosting one attribute usually weakens the other. As a result, the end product is either not sticky enough or extremely strong. The latter makes it hard to undo to adjust for mistakes or when the product is no longer useful. Now, scientists in Japan have created an intriguing, reusable glue that can instantly be stuck on and off.