In order to keep track of what a kilogram is, there exists a unit of metal which serves as the official world benchmark. This special piece of metal is known as the International Prototype Kilogram.
A replica of the IPK, kept under three glass bells.
The International Prototype Kilogram, or IPK, is a right-circular cylinder made of 90% platinum and 10% iridium. Both its height and diameter are only 39.17 mm, due to the extremely high density of platinum (almost twice as dense as lead and more than 21 times as dense as water). Iridium was added to improve the IPK’s hardness while still retaining the beneficial properties of the platinum, being extreme resistance to oxidation, average thermal and electrical conductivities, and low magnetic susceptibility.
The IPK as well as six sister copies are located in Sèvres near Paris, France. Outside of France, 42 countries have one or more national prototypes of their own.
The kilogram as a unit of mass is always equal to the mass of the International Prototype Kilogram. However, the IPK has been gradually gaining weight through adsorption of atmospheric contamination ever since it was introduced in 1875 by the BIPM (Bureau International des Poids et Mesures). As a result, the kilogram as a unit of mass is constantly getting lighter.
“You wanna fly, you got to give up the shit that weighs you down.”
– Toni Morrison, Song of Solomon
Like almost all seven base units in the International System of Units, the kilogram is a completely arbitrary value. It is not based on any existing natural value – at one point in history, it was simply decided that what we now know as a kilogram, would be a kilogram from then on. The second, however, equals one sixtieth of one sixtieth on one twenty-fourth of the time it takes for the Earth to rotate around the Sun, and is thus based on a physical, natural value, albeit not a universal one.
The metre was similarly once based on an actual object, the International Prototype Metre. In 1960 it was decided that the definition of a metre would be changed to ‘the length of the path travelled by light in a vacuum during a time interval of 1⁄299,792,458 of a second.’ Consequently, the kilogram is the only remaining value in need of an international prototype.
The seven SI base units – Kelvin, second, metre, kilogram, candela, mol and Ampère.
The IPK plays an important role in determining the values of many units, both clearly weight-related units as well as seemingly unrelated units like the lumen (luminous flux). One newton is defined as the force necessary to accelerate one kilogram at one metre per second squared. The joule is defined as that which is expended when a force of one newton acts through one metre. The watt equals one joule per second. The candela, the unit for luminous intensity, is defined as the magnitude of an electromagnetic field, in a specified direction, that has a power level of 1/683 watt per steradian at a frequency of 540 terahertz. And finally, lumen is defined as emission of candela per solid angle of one steradian (squared radian).
The long-term solution to the increasing weight of the IPK and subsequent devaluation of other units is to liberate the SI system’s dependency on the IPK by developing a practical realization of the kilogram that can be reproduced in different laboratories by following a written specification. The units of measure in such a practical realization would have their magnitudes precisely defined and expressed in terms of fundamental physical constants.
“Weight doesn’t matter. It really doesn’t. I mean, it does if you’re a model or whatever.”
– Meg Cabot, Queen of Babble
Knowledge Guild 
The precision of the metre would have boggled the mind of a Bronze Age resident, whose use of a cubit as a unit of measure was determined by the distance from a person’s elbow, to the tip of the middle finger, and of course varied considerably from person to person.
One could imagine the rise of an entire profession of long-armed buyers and short-armed sellers, but that didn’t happen, at least not on a regular basis.