You may or may not have heard, but there is an object in this world which is "the kilogram".
"Nonsense", you may say. "A kilogram is the weight of a litre of water".
"Nay", I reply, "The kilogram is a cylinder with a height and diameter of 39.17 millimetres, made of a platinum and iridium alloy (90% platinum to 10% iridium), known as the International Prototype Kilogram, which is stored in an environmentally monited safe in the lower vault of the basement of the International Bureau of Weights and Measures's House of Breteuil in Sèvres on the outskirts of Paris, with six sister copies. The vault can only be opened by three independently controlled keys (much like the urban legends concerning the formula for Coca-Cola)."
"Cor blimey!" you might reply, with a stunned expression.
"That's not all!" I respond with a grin. "The modern IPK is not the original kilogram - the original "kilogram", or the Kilogram of the Archives, was a cylinder made of solid platinum - but the IPK's mass has been found to be indestinguishable from that of the Kilogram of the Archives - which was made ninety years earlier."
"Wow. That's impressive."