The guardians of the world's most important standards of weights and measures have turned to the weird universe of quantum physics to try to resolve a dilemma.
To the bafflement of scientists, a cylinder of metal sitting in a closely-guarded strongbox that is the global benchmark for the kilogram is changing mass.
The enigma doesn't affect anyone who wants to buy 500-milligramme tablets of aspirin, half a kilo of carrots or a 50,000-tonne cruise ship.
But it poses a hefty theoretical challenge to physicists, and complicates the work of labs which need ultra-precise, always-standard measurement.
Since 1889, the kilogram has been internationally defined in accordance with a piece of metal kept at the International Bureau of Weights and Measures (known by its French acronym of BIPM), in the Paris suburb of Sevres.
Ninety-percent platinum and 10-percent iridium, the British-made cylinder was proudly deemed at its founding to be as inalienable as the stars in the sky.
It is kept under three glass cases in a safe in a protected building, the Pavillon de Breteuil.
In 1992 came a shock: the famous kilo was no longer what it should be.
Measurements made over a century showed that the prototype had changed by around 50 microgrammes -- the equivalent of a tiny grain of sand 0.4 millimetres (0.015 inches) in diameter -- compared to six other kilos also stored in Sevres.
"Actually, we're not sure whether it lost mass or gained it," Alain Picard, director of the BIPM's Mass Department, told AFP.
"The change may be to due to surface effects, loss of gas from the metal or a buildup of contaminant."
The skinnier (or fatter) kilo became more than a scientific curiosity.