'I suppose... I could try to perform some great service?' said Rincewind, with the expression of one who knows that the light at the end of the tunnel is an incoming train.

'Really? Hmm? Well, that's definitely a thought,' said Ridcully.

'What sort of services are they?'

'Oh, typically you'd be expected to, for the sake of example, go on a quest, or find the answer to some very ancient and important question - What the hell is that thing with all the legs?'

Rincewind didn't even bother to look round. The expression on Ridcully's face, as it stared over his shoulder, was quite familiar.

'Ah,' he said, 'I think I know that one.'

Magic isn't like maths. Like the Discworld itself, it follows common sense rather than logic. And nor is it like cookery. A cake's a cake. Mix the ingredients up right and cook them at the right temperature and a cake happens. No casserole requires moonbeams. No souffle ever demanded to be mixed by a virgin.

Nevertheless, those afflicted with an enquiring turn of mind have often wondered whether there are rules of magic. There are more than five hundred known spells to secure the love of another person, and they range from messing around with fern seed at midnight to doing something rather unpleasant with a rhino horn at an unspecified time, but probably not just after a meal. Was it possible (the enquir ing minds enquired) that an analysis of all these spells might reveal some small powerful common denominator, some meta-spell, some simple little equation which would achieve the required end far more simply, and incidentally come as a great relief to all rhinos?

To answer such questions Hex had been built, although Ponder Stibbons was a bit uneasy about the word 'built' in this context. He and a few keen students had put it together, certainly, but... well... sometimes he thought bits of it, strange though this sounded, just turned up.