I remember being fascinated to learn that the length of a meter used to be defined by a standard measuring rod located somewhere in France, before being redefined in terms of the distance light travels in a certain infinitesimally small fraction of a second (which in turn has been re-defined in terms of periods of atomic radiation). There was something magnificently Platonic about the idea that there existed somewhere the metre, of which all other meters are copies. Now I come to find that even to this day, a kilogram is still defined in terms of a standard prototype kept under lock and key in Sevres, France, and which mysteriously seems to have been losing mass, albeit an almost immeasurably small amount of mass. This, in turn, has prompted scientists to start talking again about finding a somewhat more abstract and thus immutable standard of mass, which in turn appears to involve determining a precise value for good old Avogadro's constant (oh, the memories of 10th-grade Chemistry...). Maybe it's my penchant for absolute standards that draws my eye to these matters. I'm just glad I'm not the one who has to do the math.