And therefore, if their edges are infinitely long, the wings must logically be infinitely big.

They may look about the right size for a butterfly's wings, but that's only because human beings have always preferred common sense to logic.

The Quantum Weather Butterfly (Papilio tempestae) is an undistinguished yellow colour, although the mandelbrot patterns on the wings are of considerable interest. Its outstanding feature is its ability to create weather.

This presumably began as a survival trait, since even an extremely hungry bird would find itself incon-venienced by a nasty localized tornado. From there it possibly became a secondary sexual characteristic, like the plumage of birds or the throat sacs of certain frogs. Look at me, the male says, flapping his wings lazily in the canopy of the rain forest. I may be an undistinguished yellow colour but in a fortnight's tone, a thousand miles away, Freak Gales Cause Road Chaos.

This is the butterly of the storms.

It flaps its wings...

This is the Discworld, which goes through space on the back of a giant turtle.

Most worlds do, at some time in their perception. It's a cosmological view the human brain seems preprogrammed to take.

On veldt and plain, in cloud jungle and silent red desert, in swamp and reed marsh, in fact in any place where something goes 'plop' off a floating log as you approach, variations on the following take place at a crucial early point in the development of the tribal mythology...

'You see dat?'

'What?'

'It just went plop off dat log.'

'Yeah? Well?'

'I reckon... I reckon... like, I reckon der world is carried on der back of one of dem.'

A moment of silence while this astrophysical hypothesis is considered, and then...