When is a kilogram not a kilogram? When it's rendered obsolete, it turns out.
The world's official kilogram -- a metal block sitting in a hermetically sealed room in Paris -- may soon be replaced by a calculation based on quantum physics.
The golf ball-sized cylinder has defined the kilogram since 1879, but aside from the fact it's nearly 140 years old, there's one big problem.
"It's so precious that people don't want to use it," says physicist Stephan Schlamminger of the National Institute of Standards and Technology in the United States.
Called Le Grand K, the block sits inside three bell jars in a climate-controlled room, separated from the public by numerous locks. Any change at all in its conditions would change its mass, either through absorption or corrosion.
It's only been taken out of its bubble a few times in history, to compare with replica blocks kept around the world.
Le Grand K won't last forever, so Dr Schlamminger wants to redefine the kilogram in reference to a constant of nature that won't ever change -- the Planck constant, a tiny number that relates to a quantum particle's wave frequency to its energy.
Until recently, physicists' equipment wasn't accurate enough to measure the Planck constant with the accuracy required to redefine the kilogram.
It's hoped that will change by July 2017, when the General Conference on Weights and Measures will meet to calculate the value of the new kilogram.
But don't expect any pleasant surprises when you step on the scales next winter. The aim is to come up with a figure as close as possible to the Le Grand K.
"It's the frustrating part about being a metrologist," says Dr Schlamminger. "If you do your job right, nobody should notice."
As for the old hunk of metal, despite being rendered useless, it's unlikely to end up in the recycling bin.
"It's such a symbol and it has such a rich history of measurement. I don't think people will just throw it in the garbage."
In 1992, it was discovered Le Grand K had either shrunk, or its replicas had increased in mass -- but as Le Grand K was literally the benchmark, no one could tell for sure which had happened. It was decided in 2011 to link the kilogram to a physical constant so this wouldn't happen again.