It used to just be defined as "exactly how much that one kilogram weight weighs over there," but (you'll never see this coming) then that weight changed how much it weighed so people were like "OH NO WHAT DO WE DO NOW" so that's what they did.
It used to just be defined as "exactly how much that one kilogram weight weighs over there," but (you'll never see this coming) then that weight changed how much it weighed so people were like "OH NO WHAT DO WE DO NOW" so that's what they did.