Horace Walpole made up the word serendipity in a letter to a friend in January 1754, and explained it with a fairy tale he had once read, The Three Princes of Serendip. The princes, he wrote, were always making discoveries, by accidents and sagacity, of things they were not in quest of. Then he gave his example. One of them worked out that a mule blind in its right eye had lately walked the same road, because the grass had been eaten only on the left side. That is not an accident. That is a man looking at a verge and reading it. Walpole’s one illustration of happy chance is a piece of detective work, and the only luck in it is that the mule happened to go that way. The word has spent more than two and a half centuries meaning the accident. The other half of the definition, the part that notices which side of the road is chewed, was there in the first letter, and it was the half doing the work.