PARIS, FRANCE: A riddle: When is a kilogramme that is no longer a kilogramme still a kilogramme?

Answer: When the chunk of metal that serves as the official international standard for the kilo, under triple lock-and-key in France since 1889, inexplicably sheds a little weight.

Very little, though. The cylinder of platinum and iridium, sitting under three concentric glass bells like a metallic round of goat's cheese, is 50 microgrammes - 0.0000017 ounces - lighter than the average of several dozen copies.

That may not seem like much, but in an era when measures of all kinds - distance, time, space - have reached levels of precision unimaginable even a generation ago, a few millionths of a kilo is a big deal.

Scientists at the International Bureau of Weights and Measures in Sevres, near Paris, where the standard is kept, are baffled.

"We do not know how this phenomenon occurred since the prototype and the copies are preserved in the same environment. They were also made at the same time and in the same manner," the Bureau's director, Richard Davis, told AFP.

But if Davis and his colleagues do not have an explanation for the missing microgrammes, they do have a solution.

Sometime before 2010, he said, the mysteriously shrinking cylinder will be replaced with a new-and-improved model, a perfect sphere measuring 93 milimetres (3.66 inches) in diameter and made of pure silicon-28.

"The advantage of silicon-28 is that it is stable. The mass does not change over time," said Davis. "That will permit us, then, to have an invariable mass for the kilogramme standard."

A kilogramme is the equivalent of 2.2 pounds - give or take a few microgrammes.