What was the original "failed" adhesive that led to the invention of Post-it Notes described as by its inventor?

Sometimes the greatest products come from accidental discoveries. In the world of innovation, the journey from laboratory mishap to market success can be fascinating. The Post-it Note, now ubiquitous in offices worldwide, began as a failed experiment that languished for years before becoming one of the most successful office products in history. Test your knowledge about this iconic product's development journey!

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A 'super-strong' adhesive that was too permanent
A 'pressure-sensitive' adhesive that was too weak
A 'heat-activated' adhesive that required impractical temperatures
A 'moisture-sensitive' adhesive that dissolved too quickly