You might think of metric values as set in stone, but in some situations, that's not the case. Take kilograms. They've long been defined as the mass of a cylinder of certain dimensions crafted from platinum and iridium, rather than in terms of a fundamental constant of nature. That cylinder is an actual object; it's known as the international prototype of the kilogram and it's housed in Sèrves, France, at the headquarters of the International Bureau of Weights and Measures (the Bureau International des Poids et Mesures, or BIPM).
Unfortunately, over time, minor variations have been measured between the weight of the international prototype and its official replicas. Metrologists have been working on the issue, but have yet to find a way to define the kilogram in a manner that correlates exactly to known constants, such as Planck's constant and Avogadro's constant [source: Brumfiel]. Until then, a kilogram's precise weight depends on a physical artifact -- although to be fair, the BIPM has the weight nailed down to the parts per billion level. However, metrologists are an exacting bunch, and they haven't been able to fully reconcile the two -- yet. Find out more about the interesting world of numbers on the next page.