The kilogram. Source
The kilogram is the metric unit of weight that is approximately equal to 2.2 pounds. It is the accepted standard in science, and is used across the world. The metric system is by far the most popular measurement system, but there is a problem with it. Because this system is so popular, everyone uses it for science, international trade, and just about everything else, which creates the need to have a reference point to accurately measure weight against. This comparison weight is the "international prototype of the kilogram" which is basically a cylinder of 90% titanium and 10% iridium. But the mass of these weights is changing.
Canada's kilogram. Source
Each of these kilogram prototype copies are kept under extremely controlled conditions. They are each in a temperature and pressure controlled room with filtered air. But it appears as if some contaminants do manage to get in. On these cylinders there can be found extremely thin layers of carbon compounds and mercury, contributing tens of micrograms to the weight of the kilogram. This effect is very little, and the impact is hard to measure because things are weighed in comparison to other things. Even though there is surface contamination that adds weight, it could still be true that these prototypes are losing weight.
The kilogram weights are cleaned periodically, but not always at the same time in all nations. This leads to very slight comparison differences and makes it hard to know exactly what a kilogram weighs. There is some concern that this change in weight could cause a problem for science and the trade of highly restricted materials, like radioactive compounds, but the International Bureau of Weights and Measures insists that the effect is so slight that it is negligible.
In the future it is likely that we will rely on fundamental forces of nature to compare weights with, something that will be more accurate than physical items. For now, however, we will use these standardized kilogram weights that are prone to slight fluctuations.