The 118-year-old cylinder that is the international prototype for the metric kilogram, kept tightly under lock and key outside Paris, is mysteriously losing weight -- if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says it appears to have lost 50 micrograms compared with the average of dozens of copies.

For scientists, the inconstant metric constant is a nuisance, threatening calculation of things such as electricity generation. But for most people, it won't mean anything. "The kilogram will stay the kilogram, and the weights you have in a weight set will all still be correct," Davis said.