To find out how old the Earth is, Clair Patterson needed to count the lead atoms in a scrap of meteorite, and for years his counts came out wrong. The extra lead wasn’t in the meteorite. It was in the air, the dust, the glassware, the water, his own hands. So he built one of the first clean rooms, and in 1953 he had his answer, about 4.55 billion years, which still stands. Then he did the unpopular thing and asked why the present was so dirty, and spent the next twenty years showing it was the lead in petrol, against an industry that did not want to hear it. The age of the world turned out to be the easy measurement. The hard one was noticing that the last thirty years had got into everything.