The lowly kilogram is the only SI unit (International System of Units) that is still defined in relation to an artifact rather than to a fundamental physical property that can be reproduced in different laboratories.
For example, the meter was once defined as an artifact (a single platinum-iridium bar with two marks on it), however, it was eventually redefined in terms of invariant, fundamental constants of nature (the distance traveled by light in space in a given duration) so that it can be reproduced in different laboratories around the world.
Since 1889, the SI system defines the magnitude of the kilogram to be equal to the mass of the International Prototype Kilogram (IPK), which is a golfball size piece of platinum/iridium alloy which is kept under two bell jars in a safe in France.
The IPK replaced an all-platinum kilogram prototype made in 1799, which in turn was constructed to be the same mass as one decimeter (one liter) of pure water at 4 C (temperature of water's most stable density point).
The US "owns" two of the forty IPK replicas made from the same batch of alloy, but periodically returns them to France for verification.
Despite careful storage, cleaning, and handling, the mass of the IPK inexplicably lost 50 µg (0.00005 g) over the last century in comparison to its official copies. That is so small, I can't give you a meaningful comparison.
There are no technical means available to determine whether or not the entire worldwide ensemble of prototypes suffers from even greater long-term trends upwards or downwards because their mass relative to an invariant of nature is unknown.
And if all that isn't enough, the kilogram is the only SI base unit with an SI prefix (kilo) as part of its name.
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