Which failed experiment at 3M led to the invention of Post-it Notes in the 1970s?

Accidental discoveries have led to some of the world's most beloved products. The journey from laboratory mishap to retail phenomenon often involves persistence, vision, and a bit of luck. This trivia question explores one such famous product development story that revolutionized offices worldwide and created an entirely new product category that we now take for granted.

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An attempt to create super-strong adhesive that accidentally produced a weak, repositionable glue
A color-changing paper experiment that resulted in yellow sheets that wouldn't properly absorb ink
A waterproof coating test that created paper which could temporarily stick to surfaces when moistened
A heat-resistant material research that produced paper scraps that would adhere to surfaces when warmed