In the scientific product design history of Post-it Notes, what was the original 'failed' product that led to their invention?

Product design in science often involves unexpected discoveries and innovations. Many laboratory tools and everyday items we use in scientific research were created through fascinating design journeys. This poll tests your knowledge about the development of a ubiquitous product that revolutionized both laboratory and office work - the humble Post-it Note, a perfect example of how product design can transform scientific practice.

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A super-strong adhesive that permanently bonded materials
A weak, pressure-sensitive adhesive originally intended to be much stronger
A heat-activated glue for laboratory sample labeling
A waterproof coating for laboratory notebooks