What is a kilogram? A seemingly easy question with a bizarre answer. The gram (1/1000th of a kilogram) is defined as the mass of one cubic centimetre of water at 0˚C, so it seems obvious that a kilogram would simply be the similar measurement of 1000 cubic centimetres of water. Stunningly, however, the kilogram is actually defined as the mass of the International Prototype of the Kilogram, a physical object that’s kept at the International Bureau of Weights and Measures in France. Also known as La Grande K or the Big K, this object had 6 sister copies, so that authorities can check to be sure that the kilogram has not changed in mass. This bizarre way to define the metric system’s base unit of mass is even weirder, as many other units (from newtons for force to volts for voltage) are defined in reference to the kilogram.
