For, as the world tumbles lazily, it is revealed as the Discworld - flat, circular, and carried through space on the back of four elephants who stand on the back of Great A'tuin, the only turtle ever to feature on the Hertzsprung-Russell Diagram, a turtle ten thousand miles long, dusted with the frost of dead comets, meteor-pocked, albedo-eyed. No one knows the reason for all this, but it is probably quantum.