Post-it notes were invented by accident.
Why it is true
In 1968, scientist Spencer Silver was trying to develop a strong adhesive for 3M. Instead, he created a weak adhesive that could be removed and reapplied. Years later, colleague Arthur Fry used it to mark pages in his hymn book, and the product became a global success.
Source: Invention History
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.