Weight (or, better yet, mass)
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Measuring the mass of something is more difficult than that of length and time. We have a secondary standard based on an atomic scale, which uses the carbon-12 atom. This isotope of carbon is given a mass equal to 12 atomic mass units, where an atomic mass unit (1 u) is equal to 1.6605402 X 10^-27kg. This number is uncertain to about 1 part in a million - not too bad at all. The problem is that an atom of carbon-12 is not particularly convenient to use as a standard - we would prefer something a bit larger. In principle we could assemble the right number of carbon-12 atoms to match the mass of a kilogram (Do you know how many it would take? It's related to a very famous number.) The difficulty is in counting the atoms. Until a more accurate means can be found, we rely on the standard kilogram as our unit of mass. This standard is based on a volume of pure water. One cubic centimeter of water is assigned a mass of 1 gram, and 1,000 cubic centimeters (a 10 cm x 10 cm x 10 cm cube of water) a mass of 1 kilogram. As you can imagine, such a standard would be difficult to handle, so an equivalent standard, in the form of a platinum- iridium cylinder, is defined as the standard kilogram. This standard is kept at the International Bureau of Weights and Measures near Paris, to which copies are compared and kept in countries around the world. The U.S. copy is kept in a vault at NIST in Boulder, Colorado.

How is mass different from weight? (Find out and let me know).