The studs down the middle of a British road are cat’s eyes, Percy Shaw’s invention from 1934: two small glass reflectors in a rubber dome, set in an iron base. The clever part happens when a car drives over one. The rubber gives, the glass sinks into the base, and a rubber lip wipes it on the way down, often through rainwater that has pooled there. So the stud is cleaned by the very traffic it is there to guide, and cleaned best on wet nights, which are the nights it is most needed. Shaw told the story of where the idea came from in more than one way over the years. The wiper never changed.