Uropygium n. (from _Ouranos_, sky + _pyx_, fire + _-ium_)
an opalescent gray mineral ore from which blue sky is refined. The painter and naturalist Rene Magritte, often called "The Audubon of Weather," documented the mining and smelting of uropygium in a series of paintings entitled "Ceci n'est pas une pyg." The two best known of these studies depicts the most critical stages in the refining process, when impurities resembling bowler hats and green apples boil to the surface and can be skimmed off, leaving unblemished blue.
Recently three other paintings in the series have come to light:
1) "Ceci n'est pas une Jeep" records miners shoveling raw uropygium from a coastal fogbank into a dumptruck.
2) "Cela, c'est pot-au-feu" shows forklifts carrying bright blue cubes of refined sky to the construction site, where some will be winched directly into the firmament. The azure blocks stacked in the foreground await the return of the cloud-insertion crew from their coffee break, which has stretched on for some months now.
3) "Say hy a kutake" represents the almost-completed sky, an azure structure with a single block, the keystone, missing. In the black cube where the keystone will soon be set, a comet with its tail fanned wide is clearly visible.
from The Dictionary of Improbable Speech