The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight... the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies. "The mystery is that they 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," he said. "We don't really have a good hypothesis for it." The kilogram's uncertainty could affect even countries that don't use the metric system - it is the ultimate weight standard for the U.S. customary system, where it equals 2.2 pounds. For scientists, the inconstant metric constant is a nuisance, threatening calculation of things like electricity generation.