The place where the story happened was a world on the back of four elephants perched on the shell of a giant turtle. That's the advantage of space. It's big enough to hold practically anything, and so, eventually, it does.

People think that it is strange to have a turtle ten thousand miles long and an elephant more than two thousand miles tall, which just shows that the human brain is ill-adapted for thinking and was probably originally designed for cooling the blood. It believes mere size is amazing.

There's nothing amazing about size. Turtles are amazing, and elephants are quite astonishing. But the fact that there's a big turtle is far less amazing than the fact that there is a turtle anywhere.

The reason for the story was a mix of many things. There was humanity's desire to do forbidden deeds merely because they were forbidden. There was its desire to find new horizons and kill the people who live beyond them. There were the mysterious scrolls. There was the cucumber. But mostly there was the knowledge that one day, quite soon, it would be all over.

"Ah, well, life goes on," people say when someone dies. But from the point of view of the person who has just died, it doesn't. It's the universe that goes on. Just as the deceased was getting the hang of everything it's all whisked away, by illness or accident or, in one case, a cucumber. Why this has to be is one of the imponderables of life, in the face of which people either start to pray... or become really, really angry.

The beginning of the story happened tens of thousands of years ago, on a wild and stormy night, when a speck of flame came down the mountain at the centre of the world. It moved in dodges and jerks, as if the unseen person carrying it was sliding and falling from rock to rock. At one point the line became a streak of sparks, ending in a snowdrift at the bottom of a crevasse. But a hand thrust up through the snow held the smoking embers of the torch, and the wind, driven by the anger of the gods, and with a sense of humour of its own, whipped the flame back into life... And, after that, it never died.

The end of the story began high above the world, but got lower and lower as it circled down towards the ancient and modern city of Ankh-Morpork, where, it was said, anything could be bought and sold — and if they didn't have what you wanted they could steal it for you.