Probably no other world in the multiverse has warehouses for things which only exist in potentia, but the pork futures warehouse in Ankh-Morpork is a product of the Patrician’s rules about baseless metaphors, the literal-mindedness of citizens who assume that everything must exist somewhere, and the general thinness of the fabric of reality around Ankh, which is so thin that it’s as thin as a very thin thing. The net result is that trading in pork futures—in pork that doesn’t exist yet—led to the building of the warehouse to store it in until it does.

Terry Pratchett, Thud!