Serendipity means a "fortunate happenstance" or "pleasant surprise". It was coined by Horace Walpole in 1754. In a letter he wrote to a friend, Walpole explained an unexpected discovery he had made by reference to a Persian fairy tale, The Three Princes of Serendip. The princes, he told his correspondent, were "always making discoveries, by accidents and sagacity, of things which they were not in quest of". The notion of serendipity is a common occurrence throughout the history of scientific innovation such as Alexander Fleming's accidental discovery of penicillin in 1928, the invention of the microwave oven by Percy Spencer in 1945, and the invention of the Post-it note by Spencer Silver in 1968.