In September 1928 Alexander Fleming came back from a summer holiday to the culture plates he had left stacked on his bench, and a mould had got into one of them. Around the mould the staphylococci had died back in a clear ring. A contaminated plate is a failed experiment, and the ordinary thing to do with a failed experiment is wash it. Fleming kept it, and wrote later that one sometimes finds what one is not looking for. He never managed to purify the stuff; it took Howard Florey and Ernst Chain at Oxford, a decade on, to turn penicillin into a medicine, and the three shared the Nobel prize in 1945. But the first step was a man declining to tidy up. Half of discovery is being willing to call a mistake a result.