When is a Kilogram Not a Kilogram?

Evidently, if a kilogram sits around doing nothing for too long, it begins to lose weight. Or rather, mass. The 118-year-old cylinder that is the international standard for the metric mass, kept tightly under lock and key outside Paris, has mysteriously lost as much as 50 micrograms.

Made of a platinum-iridium alloy, the cylinder was cast in 1889 and measures 1.54 inches in height and diameter. Of course, it has lost mass only in relation to the 80 or so reference kilograms that exist around the world. It could be that, locked away as it's been for over a hundred years, it has remained the same and the others have gained weight.

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