I came accross this fascinating article, where...
"These silicon spheres, manufactured by metrologists at Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) as part of efforts to alleviate dependence on the International Prototype Kilogram, have properties that are as close to truly exact as measurable properties are likely to be for a long time to come: They weigh 1.0000000 kilograms, are smooth to the nearest 0.0000000003 meter, and are round to within 0.000000050 meters. One of them is even monoisotopic." Via boing2.
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