The broom whisked down the corridor raising a great cloud of dust which, if you looked hard at it, seemed somehow to be sucked back into the broomstick. If you looked even harder you'd see that the broom handle had strange markings on it, which were not so much carved as clinging and somehow changed shape as you watched.

But no one looked.

Esk sat at one of the high deep windows and stared out over the city. She was feeling angrier than usual, so the broom attacked the dust with unusual vigour. Spiders ran desperate eight-legged dashes for safety as ancestral cobwebs disappeared into the void. In the walls mice clung to each other, legs braced against the inside of their holes. Woodworm scrabbled in the ceiling beams as they were drawn, inexorably, backwards down their tunnels.

“'You can really clean up',” said Esk. “Huh!”

There were some good points, she had to admit. The food was simple but there was plenty of it, and she had a room to herself somewhere in the roof and it was quite luxurious because here she could lie in until five a. m., which to Granny's way of thinking was practically noon. The work certainly wasn't hard. She just started sweeping until the staff realised what was expected of it, and then she could amuse herself until it was finished. If anyone came the staff would immediately lean itself nonchalantly against a wall.

But she wasn't learning any wizardry. She could wander into empty classrooms and look at the diagrams chalked on the board, and on the floor too in the more advanced classes, but the shapes were meaningless. And unpleasant.

They reminded Esk of the pictures in Simon's book. They looked alive.

She gazed out across the rooftops of Ankh-Morpork and reasoned like this: writing was only the words that people said, squeezed between layers of paper until they were fossilized. Fossils were well-known on the Discworld, great spiralled shells and badly-constructed creatures that were left over from the time when the Creator hadn't really decided what He wanted to make and was, as it were, just idly messing around with the Pleistocene). And the words people said were just shadows of real things. But some things were too big to be really trapped in words, and even the words were too powerful to be completely tamed by writing.

So it followed that some writing was actually trying to become things. Esk's thoughts became confused things at this point, but she was certain that the really magic words were the ones that pulsed angrily, trying to escape and become real.

They didn't look very nice.

But then she remembered the previous day.

It had been rather odd. The University classrooms were designed on the funnel principle, with tiers of seats - polished by the bottoms of the Disc's greatest mages - looking precipitously down into a central area where there was a workbench, a couple of blackboards and enough floor space for a decent-sized instructional octogram. There was a lot of dead space under the tiers and Esk had found it a quite useful observation post, peering around between the apprentice wizards' pointy boots at the instructor. It was very restful, with the droning of the lecturers drifting over her as gently as the buzzing of the slightly zonked bees in Granny's special herb garden. There never seemed to be any practical magic, it always seemed to be just words. Wizards seemed to like words.

But yesterday had been different. Esk had been sitting in the dusty gloom, trying to do even some very simple magic, when she heard the door open and boots clump across the floor. That was surprising in itself. Esk knew the timetable, and the Second Year students who normally occupied this room were down for Beginners' Dematerialisation with Jeophal the Spry in the gym. (Students of magic had little use for physical exercise; the gym was a large room lined with lead and rowan wood, where neophytes could work out at High magic without seriously unbalancing the universe, although not always without seriously unbalancing themselves. Magic had no mercy on the ham-fisted. Some clumsy students were lucky enough to walk out, others were removed in bottles.)

Esk peeped between the slats. These weren't students, they were wizards. Quite high ones, to judge by their robes. And there was no mistaking the figure that climbed on to the lecturer's dais like a badlystrung puppet, bumping heavily into the lectern and absent-mindedly apologising to it. It was Simon. No one else had eyes like two raw eggs in warm water and a dose bright red from blowing. For Simon, the pollen count always went to infinity.

It occurred to Esk that, minus his general allergy to the whole of Creation and with a decent haircut and a few lessons in deportment, the boy could look quite handsome. It was an unusual thought, and she squirrelled it away for future consideration.

When the wizards had settled down, Simon began to talk. He read from notes, and every time he stuttered over a word the wizards, as one man, without being able to stop themselves, chorused it for him.

After a while a stick of chalk rose from the lectern and started to write on the blackboard behind him. Esk had picked up enough about wizard magic to know that this was an astounding achievement- Simon had been at the University for a couple of weeks, and most students hadn't mastered Light Levitation by the end of their second year.

The little white stub skittered and squeaked across the blackness to the accompaniment of Simon's voice. Even allowing for the stutter, he was not a very good speaker. He dropped notes. He corrected himself. He ummed and ahhed. And as far as Esk was concerned he wasn't saying anything very much. Phrases filtered down to her hiding place. “Basic fabric of the universe” was one, and she didn't understand what that was, unless he meant denim, or maybe flannelette. “Mutability of the possibility matrix” she couldn't guess at all.

Sometimes he seemed to be saying that nothing existed unless people thought it did, and the world was really only there at all because people kept on imagining it. But then he seemed to be saying that there was lots of worlds, all nearly the same and all sort of occupying the same place but all separated by the thickness of a shadow, so that everything that ever could happen would have somewhere to happen in.

(Esk could get to grips with this. She had half-suspected it ever since she cleaned out the senior wizards' lavatory, or ratherwhile the staff got on with the job while Esk examined the urinals and, with the assistance of some half-remembered details of her brothers in the tin bath in front of the fire at home, formulated her unofficial General Theory of comparative anatomy. The senior wizards' lavatory was a magical place, with real running water and interesting tiles and, most importantly, two big silver mirrors fixed to opposite walls so that someone looking into one could see themselves repeated again and again until the image was too small to see. It was Esk's first introduction to the idea of infinity. More to the point, she had a suspicion that one of the mirror Esks, right on the edge of sight, was waving at her.)

There was something disturbing about the phrases Simon used. Half the time he seemed to be saying that the world was about as real as a soap bubble, or a dream.

The chalk shrieked its way across the board behind him. Sometimes Simon had to stop and explain symbols to the wizards, who seemed to Esk to be getting excited at some very silly sentences. Then the chalk would start again, curving across the darkness like a comet, trailing its dust behind it.

The light was fading out of the sky outside. As the room grew more gloomy the chalked words glowed and the blackboard appeared to Esk to be not so much dark as simply not there at all, but just a square hole cut out of the world.

Simon talked on, about the world being made up of tiny things whose presence could only be determined by the fact that they were not there, little spinning balls of nothingness that magic could shunt together to make stars and butterflies and diamonds. Everything was made up of emptiness.

The funny thing was, he seemed to find this fascinating.

Esk was only aware that the walls of the room grew as thin and insubstantial as smoke, as if the emptiness in them was expanding to swallow whatever it was that defined them as walls, and instead there was nothing but the familiar cold, empty, glittering plain with its distant worn hills, and the creatures that stood as still as statues, looking down. There were a lot more of them now. They seemed for all the world to be clustering like moths around a light.

One important difference was that a moth's face, even close up, was as friendly as a bunny rabbit's compared to the things watching Simon.

Then a servant came in to light the lamps and the creatures vanished, turning into perfectly harmless shadows that lurked in the corners of the room.

At some time in the recent past someone had decided to brighten the ancient corridors of the University by painting them, having some vague notion that Learning Should Be Fun. It hadn't worked. It's a fact known throughout the universes that no matter how carefully the colours are chosen, institutional decor ends up as either vomit green, unmentionable brown, nicotine yellow or surgical appliance pink. By some little understood process of sympathetic resonance, corridors painted in those colours always smell slightly of boiled cabbage-even if no cabbage is ever cooked in the vicinity.

Somewhere in the corridors a bell rang. Esk dropped lightly from her windowsill, grabbed the staff and started to sweep industriously as doors were flung open and the corridors filled with students. They streamed past her on two sides, like water around a rock. For a few minutes there was utter confusion. Then doors slammed, a few laggard feet pattered away in the distance, and Esk was by herself again.

Not for the first time, Esk wished that the staff could talk. The other servants were friendly enough, but you couldn't talk to them. Not about magic, anyway.

She was also coming to the conclusion that she ought to learn to read. This reading business seemed to be the key to wizard magic, which was all about words. Wizards seemed to think that names were the same as things, and that if you changed the name, you changed the thing. At least, it seemed to be something like that ....

Reading. That meant the library. Simon had said there were thousands of books in it, and amongst all those words there were bound to be one or two she could read. Esk put the staff over her shoulder and set off resolutely for Mrs Whitlow's office.

She was nearly there when a wall said “Psst!” When Esk stared at it it turned out to be Granny. It wasn't that Granny could make herself invisible, it was just that she had this talent for being able to fade into the foreground so that she wasn't noticed.

“How are you getting on, then?” asked Granny. “How's the magic coming along?”

“What are you doing here, Granny?” said Esk.

“Been to tell Mrs Whitlow her fortune,” said Granny, holding up a large bundle of old clothes with some satisfaction. Her smile faded under Esk's stern gaze.

“Well, things are different in the city,” she said. “City people are always worried about the future, it comes from eating unnatural food. Anyway,” she added, suddenly realising that she was whining, “Why shouldn't I tell fortunes?”

“You always said Hilta was playing on the foolishness of her sex,” said Esk. “You said that them as tell fortunes should be ashamed of themselves, and anyway, you don't need old clothes.”

“Waste not, want not,” said Granny primly. She had spent her entire life on the old-clothes standard and wasn't about to let temporary prosperity dislodge her: “Are you getting enough to eat?”