Interesting fact I learned today.

there is such a thing as THE Kilogram. As in the actual object by the which the weight 'one kilogram' is defined.

It's been kept in a vault in Paris since 1889. Other countries have their own copy of the main kilogram but the Paris one is the main one.

BUT "on occasions when the prototype has been compared with its closest relatives - the 40 or so duplicate kilograms kept in various labs worldwide - it has emerged that its mass has drifted by at least 100 micrograms, the mass of roughly half a sugar crystal."

I found this interesting, did you?