What exactly is a kilogram? The best way to find out is to peer into a vault maintained by the International Bureau of Weights and Measures in France. Inside that vault rests a cylindrical chunk of metal known as the kilogram "standard." It’s been there since the late 1800s, and it serves as the authoritative kilogram.
Recently, though, scientists keeping tabs on the "standard" observed that it no longer weighs the same as the dozens of replicas distributed across the world. It appears the "standard" has gained a little weight.
Richard Davis, physicist and former head of the Mass Section at the International Bureau of Weights and Measures explains how this happened, and what he and his colleagues are doing about it.