A kilogram isn't what it used to be

The agreed-on international physical standard for the kilogram, a cylinder kept in airtight conditions in a locked vault in France, has lost weight. Losing 50 micrograms isn't much - about the weight of a fingerprint, as one writer put it - but it's baffling. As physicist Richard Davis of the International Bureau of Weights and Measures put it, the master kilogram and its many copies used around the world "were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart. We don't really have a good hypothesis for it."