' The currently accepted answer is the mass of a cylinder of platinum and iridium called the International Prototype Kilogram.

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Because of this, the actual International Prototype Kilogram (yes, there does only seem to be one in existence) is stored in a vault in Sevres, France, limiting the lay person's ability to determine the actual weight of a kilogram.

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The problem with that definition, of course, is that not just anybody can measure it -- since most of us don't have an International Prototype Kilogram laying around, especially since every time the thing is picked up a few atoms rub off of it making it a little bit lighter.

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