They've been kept in controlled conditions in different locations all over the world. Every 50 years they have a reunion and compare them.
And they keep changing weight.
At the last major kilogram comparison done in and around 1990, some copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms. The United States' No. 20 was 18 micrograms heavier.
There was no way to tell which was changing: Le Grand K, its copies or both.
Perhaps the platinum in the cylinders was sopping up mercury from the atmosphere. Maybe dissolved gas was escaping from the cylinders. One idea was that cleaning the cylinders with distilled water and ether had altered their weights.
"Nobody has a really good idea why," said Davis of the International Bureau of Weights and Measures. "It's all speculation."