Fact
The kilogram stopped being a physical object in 2019
For 130 years it was a lump of metal in a vault near Paris - and it was losing weight.
The International Prototype Kilogram drifted measurably against its own copies, meaning the definition of mass was unstable. In 2019 the kilogram was redefined in terms of the Planck constant, realised through a Kibble balance. Every SI base unit is now defined by physical constants rather than artefacts.