The standard kilogram has lost weight since 1889.
Since 1889, a platinum cylinder in a vault beneath a 17th century pavilion on the outskirts of Paris has been the international standard for the kilogram.
But the International Bureau of Weights and Measures have noticed that the bar has lost approximately 50 micrograms, roughly equal to a grain of sand.
No one knows for sure what went wrong with Le Grand K, but some theorize it lost weight from being cleaned.
A new definition of one kilogram mass based on Planck's constant will establish a standard based on natural laws that can be calculated mathematically.