When it's not, er, a kilogram.
Particularly if the kilogram in question is the kilogram against which all other kilograms have been measured for the last 130 years.
That singular beauty - a cyclinder composed of platinum-iridium - is kept in a vault in France, at the International Bureau of Weights and Measures.
It's something of a relic these days. Once upon a time there were seven artifacts in the weights and measures family.
Along with the kilogram, there was the candela, which measured luminous intensity, the kelvin (temperature), the metre (length), the ampere (electric current), the mole (substance) and the second (time).
Of all of them, only the kilogram still exists. The problem is, over 130 years, it's changed, for a range of reasons that will only make sense to people with a quantum physics degree.