The Kilogram doesn’t weigh a kilogram any more.
Together with six other units – metre, second, ampere, kelvin, mole, and candela – the kilogram, a unit of mass, is part of the International System of Units (SI) that is used as a basis to express every measurable object or phenomenon in nature in numbers. This unit’s current definition is based on a small platinum and iridium cylinder, known as “le grand K”, whose mass is exactly one kilogram. The cylinder was crafted in 1889 and, since then, has been kept safe under three glass bell jars in a high-security vault on the outskirts of Paris.
There is one problem: the current standard kilogram is losing weight. About 50 micrograms, at the latest check. Enough to be different from its once-identical copies stored in laboratories around the world.
Fascinating; sometimes we just take certain bits of knowledge for granted, such as not realising that the kilogram like any scale of measurement is ultimately referencing a certain object.