The truth is that even big collections of ordinary books distort space, as can readily be proved by anyone who has been around a really old-fashioned secondhand bookshop, one that looks as though they were designed by M. Escher on a bad day and has more stairways than storeys and those rows of shelves which end in little doors that are surely too small for a full-sized human to enter. The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read.

Most species do their own evolving, making it up as they go along, which is the way Nature intended.

Some people believe that when you die, you cross the River of Death and have to pay the ferryman. People don’t seem to worry about that these days. Perhaps there’s a bridge now.

And it came to pass that in time the Great God Om spake unto Brutha, the Chosen One: "Psst!

Sergeant Colon of the Ankh-Morpork City Guard was on duty. He was guarding the Brass Bridge, the main link between Ankh and Morpork. From theft. When it came to crime prevention, Sergeant Colon found it safest to think big.

It was all very well going on about pure logic and how the universe was ruled by logic and the harmony of numbers, but the plain fact of the matter was that the Disc was manifestly traversing space on the back of a giant turtle and the gods had a habit of going round to atheists' houses and smashing their windows.

It’s quite easy to accidentally overhear people talking downstairs if you hold an upturned glass to the floorboards and accidentally put your ear to it.