In a corridor of the Clarendon Laboratory in Oxford there is a bell that has been ringing since 1840. Two small brass bells sit under two dry piles, an early kind of battery, and a metal ball about four millimetres across hangs between them, pulled to one bell, then pushed to the other, about twice a second. It has rung more than ten billion times, and you can’t hear any of them: it stands behind two layers of glass. Nobody knows exactly what the piles are made of, because the men who built them left no record and the outsides are sealed in sulfur. The only way to find out is to open them, which would stop the bell. So the physicists are waiting for it to stop on its own. The answer has been sitting in the corridor for a hundred and eighty-six years, and every day the ringing wins.