Because a lot of science is really about this non-existent world of thought experiments, our understanding of science must concern itself with worlds of the imagination as well as with worlds of real­ity. Imagination, rather than mere intelligence, is the truly human quality. And what better world of the imagination to start from than Discworld? Discworld is a consistent, well-developed universe with its own kinds of rules, and convincingly real people live on it despite the substantial differences between their universe's rules and ours. Many of them also have a thoroughgoing grounding in 'common sense', one of science's natural enemies.

Appearing regularly within the Discworld canon are the buildings and faculty of Unseen University, the Discworld's premier col­lege of magic. The wizards are a lively bunch, always ready to open any door that has 'This door to be kept shut' written on it or pick up anything that has just started to fizz. It seemed to us that they could be useful ...

If we, or they, compare Discworld's magic to Roundworld sci­ence, the more similarities and parallels we find. Clearly, as the wizards of Unseen University believe, this world is a parody of the Discworld one. And when we didn't discover those, we found that the differences were very revealing. Science takes on a new character when you stop asking questions like 'What does newt DNA look like?' and instead ask 'I wonder how the wizards would react to this way of thinking about newts?'

There is no science as such on Discworld. So we have put some there. By magical means, the wizards on Discworld must be led to create their own brand of science, some kind of pocket universe' in which magic no longer works, but rules do. Then, as the wizards learn to understand how the rules make interesting things happen -rocks, bacteria, civilizations, we watch them watching ... well, us. It's a sort of recursive thought experiment, or a Russian doll wherein the smaller dolls are opened up to find the largest doll inside.

And then we found that ... ah, but that is another story.

TP, IS, & JC, DECEMBER 1998

PS We have, we are afraid, mentioned in the ensuing pages Schrodinger's Cat, the Twins Paradox, and that bit about shining a torch ahead of a spaceship travelling at the speed of light. This is because, under the rules of the Guild of Science Writers, they have to be included. We have, however, tried to keep them short.

We've managed to be very, very brief about the Trousers of Time, as well.

SOME QUESTIONS SHOULD NOT BE ASKED. However, someone always does.

'How does it work?' said Archchancellor Mustrum Ridcully, the Master of Unseen University.

This was the kind of question that Ponder Stibbons hated almost as much as 'How much will it cost?' They were two of the hardest questions a researcher ever had to face. As the university's de facto head of magical development, he especially tried to avoid questions of finance at all costs.

'In quite a complex way.' he ventured at last.

'Ah.'

'What I'd like to know,' said the Senior Wrangler, 'is when we're going to get the squash court back.'

'You never play, Senior Wrangler,' said Ridcully, looking up at the towering black construction that now occupied the centre of the old university court.

'I might want to one day. It'll be damn hard with that thing in the way, that's my point. We'll have to completely rewrite the rules.'

Outside, snow piled up against the high windows. This was turn­ing out to be the longest winter in living memory, so long, in fact, that living memory itself was being shortened as some of the older citizens succumbed. The cold had penetrated even the thick and ancient walls of Unseen University itself, to the general concern and annoyance of the faculty. Wizards can put up with any amount of deprivation and discomfort, provided it is not happening to them.

And so, at long last, Ponder Stibbons's project had been author­ized. He'd been waiting three years for it. His plea that splitting the thaum would push back the boundaries of human knowledge had fallen on deaf ears; the wizards considered that pushing back the boundaries of anything was akin to lifting up a very large, damp stone. His assertion that splitting the thaum might significantly increase the sum total of human happiness met with the rejoinder that everyone seemed pretty happy enough already.

Finally he'd ventured that splitting the thaum would produce vast amounts of raw magic that could very easily be converted into cheap heat. That worked. The Faculty were lukewarm on the sub­ject of knowledge for knowledge's sake, but they were boiling hot on the subject of warm bedrooms.

Now the other senior wizards wandered around the suddenly-cramped court, prodding the new thing. Their Archchancellor took out his pipe and absent-mindedly knocked out the ashes on its matt black side.

'Um ... please don't do that, sir,' said Ponder.

'Why not?'

'There might be ... it might... there's a chance that...' Ponder stopped. 'It will make the place untidy, sir,' he said.

'Ah. Good point. So it's not that the whole thing might explode, then?'

'Er ... no, sir. Haha,' said Ponder miserably. 'It'd take a lot more than that, sir...'

There was a whack as a squash ball ricocheted off the wall, rebounded off the casing, and knocked the Archchancellor's pipe out of his mouth.

'That was you. Dean,' said Ridcully accusingly. 'Honestly, you fellows haven't taken any notice of this place in years and suddenly you all want to, Mr Stibbons? Mr Stibbons?'

He nudged the small mound that was the hunched figure of the University's chief research wizard. Ponder Stibbons uncurled slightly and peered between his fingers.

'I really think it might be a good idea if they stopped playing squash, sir,' he whispered.

'Me too. There's nothing worse than a sweaty wizard. Stop it, you fellows. And gather round. Mr Stibbons is going to do his pres­entation.' The Archchancellor gave Ponder Stibbons a rather sharp look. 'It is going to be very informative and interesting, isn't it, Mister Stibbons. He's going to tell us what he spent AM$55,879.45p on.'

'And why he's ruined a perfectly good squash court,' said the Senior Wrangler, tapping the side of the thing with his squash racket.

'And if this is safe? said the Dean. 'I'magainst dabbling in physics,'

Ponder Stibbons winced.

'I assure you, Dean, that the chances of anyone being killed by the, er, reacting engine are even greater than the chance of being knocked down while crossing the street,' he said.

'Really? Oh, well ... all right then.'

Ponder reconsidered the impromptu sentence he'd just uttered and decided, in the circumstances, not to correct it. Talking to the senior wizards was like building a house of cards; if you got anything to stay upright, you just breathed out gently and moved on.

Ponder had invented a little system he'd called, in the privacy of his head, Lies-to-Wizards. It was for their own good, he told him­self. There was no point in telling your bosses everything; they were busy men, they didn't want explanations. There was no point in bur­dening them. What they wanted was little stories that they felt they could understand, and then they'd go away and stop worrying.

He'd got his students to set up a small display at the far end of the squash court. Beside it, with pipes looping away through the wall into the High Energy Magic building next door, was a termi­nal to HEX, the University's thinking engine. And beside that was a plinth on which was a very large red lever, around which someone had tied a pink ribbon.