The Post-it Note was created by accident. A scientist discovered a weak reusable adhesive whilst trying to create a super strong one. Classic Post-its are yellow because the paper used to test the adhesive was yellow.
They were originally sold under the name Press 'n Peel.
It was a very simple invention that only became important because it was invented.
They are now made with plant-based adhesive (67% plant-based adhesive by weight).