From 1889 until 2019 the kilogram was a thing: a small cylinder of platinum and iridium, about the size of a golf ball, kept under three glass bell jars in a vault outside Paris. Every few decades it was brought out and weighed against its official copies, and over a century they drifted apart by about fifty micrograms, fifty millionths of a gram. The odd part is that nobody could say which side had moved. The cylinder weighed exactly one kilogram by definition, whatever happened to it; if it shed a few atoms, then in the eyes of the law everything else on Earth had got slightly heavier. In May 2019 the definition was moved onto the Planck constant, a number that doesn’t sit in a vault, and the cylinder is now just very nearly a kilogram, like everything else.