Fundamental Constants

You know, I like the definition of a kilogram as being the mass of one liter of pure water but there are the problems related to exactly reproducing the physical conditions so the genuises at the heart of the metric system had to have physical artifacts to which all samples would be referenced. This article is an amazing admission of the limitations of having physical reference objects.

http://www.cnn.com/2007/TECH/science/09/12/shrinking.kilogram.ap/index.html