The trolls had clapped their huge hands over their ears, but Lady LeJean merely stood with her hands on her hips, head on one side, until the last echo died away. 'All correct, we see,' she said. 'What?' said Jeremy. He'd been thinking: perhaps a vampire, then? 'You keep all your clocks at the right time,' said Lady LeJean. 'You're very particular about that, Mr Jeremy?'

'A clock that doesn't tell the right time is ... wrong,' said Jeremy. Now he was wishing she'd go away. Her eyes were worrying him. He'd heard about people having grey eyes, and her eyes were grey, like the eyes of a blind person, but she was clearly looking at him and through him. 'Yes, there was a little bit of trouble over that, wasn't there?' said Lady LeJean. 'I... I don't ... I don't ... don't know what you're-'

'At the Clockmakers' Guild? Williamson, who kept his clock five minutes fast? And you-'

'I am much better now,' said Jeremy stiffly. 'I have medicine. The Guild was very kind. Now please go away.'

'Mr Jeremy, we want you to build us a clock that is accurate.'

'All my clocks are accurate,' said Jeremy, staring at his feet. He wasn't due to take his medicine for another five hours and seventeen minutes, but he was feeling the need for it now. 'And now I must ask-'

'How accurate are your clocks?'

'Better than a second in eleven months,' said Jeremy promptly. 'That is very good?'

'Yes.' It had been very good. That was why the Guild had been so understanding. Genius is always allowed some leeway, once the hammer has been pried from its hands and the blood has been cleaned up. 'We want much better accuracy than that.'

'It can't be done.'

'Oh? You mean that you can't do it?'

'No, I can't. And if I can't, then neither can any other clockmaker in the city. I'd know about it if they could!'

'So proud? Are you sure?'

'I'd know.' And he would. He'd know for certain. The candle clocks and the water clocks... they were toys, which he kept out of a sort of respect for the early days of timekeeping, and even then he'd spent weeks experimenting with waxes and buckets and had turned out primitive clocks that you could, well, very nearly set your watch by. It was okay that they couldn't be that accurate. They were simple, organic things, parodies of time. They didn't grind across his nerves. But a real clock... well, that was a mechanism, a thing of numbers, and numbers had to be perfect. She put her head on one side again. 'How do you test to that accuracy?' she said. They'd often asked him that in the Guild, once his talent had revealed itself. He hadn't been able to answer the question then, either, because it didn't make sense. You built a clock to be accurate. A portrait painter painted a picture. If it looked like the subject, then it was an accurate picture. If you built the clock right, it would be accurate. You didn't have to test it. You'd know. 'I'd know,' he said. 'We want you to build a clock that is very accurate.'

'How accurate?'

'Accurate.'

'But I can only build to the limit of my materials,' said Jeremy. 'I have... developed certain techniques, but there are things like... the vibration of the traffic in the street, little changes in temperature, that sort of thing.' Lady LeJean was now inspecting a range of fat imp-powered watches. She picked one up and opened the back. There was the tiny saddle, and the pedals, but they were forlorn and empty. 'No imps?' she said. 'I keep them for historical interest,' said Jeremy. 'They were barely accurate to a few seconds a minute, and they'd stop completely overnight. They were only any good if your idea of accuracy was “around two-ish”.' He grimaced when he used the term. It felt like hearing fingernails on a blackboard. 'How about invar?' said the lady, still apparently inspecting the museum of clocks. Jeremy looked shocked. 'The alloy? I didn't think anyone outside the Guild knew about that. And it is very expensive. Worth a lot more than its weight in gold.' Lady LeJean straightened up. 'Money is no object,' she said. 'Would invar allow you to reach total accuracy?'

'No. I already use it. It's true that it is not affected by temperature, but there are always... barriers. Smaller and smaller interferences become bigger and bigger problems. It's Xeno's Paradox.'

'Ah, yes. He was the Ephebian philosopher who said you couldn't hit a running man with an arrow, wasn't he?' said the lady. 'In theory, because-'

'But Xeno came up with four paradoxes, I believe,' said Lady LeJean. 'They involved the idea that there is such a thing as the smallest possible unit of time. And it must exist, mustn't it? Consider the present. It must have a length, because one end of it is connected to the past and the other is connected to the future, and if it didn't have a length then the present couldn't exist at all. There would be no time for it to be the present in.' Jeremy was suddenly in love. He hadn't felt like this since he'd taken the back off the nursery clock when he was fourteen months old. 'Then you're talking about... the famous “tick of the universe”,' he said. 'And no gear cutter could possibly make gears that small...'

'It depends on what you would call a gear. Have you read this?' Lady LeJean waved a hand at one of the trolls, who lumbered over and dropped an oblong package on the counter. Jeremy undid it. It contained a small book. 'Grim Fairy Tales?' he said. 'Read the story about the glass clock of Bad Schüschein,' said Lady LeJean. 'Children's stories?' said Jeremy. 'What can they tell me?'

'Who knows? We will call again tomorrow,' said Lady LeJean, 'to hear about your plans. In the meantime, here is a little token of our good faith.' The troll laid a large leather bag on the counter. It clinked with the heavy, rich clink of gold. Jeremy didn't pay it a great deal of attention. He had quite a lot of gold. Even skilled clockmakers came to buy his clocks. Gold was useful because it gave him the time to work on more clocks. These earned him more gold. Gold was, more or less, something that occupied the space between clocks. 'I can also obtain invar for you, in large quantities,' she said. 'That will be part of your payment, although I agree that even invar will not serve your purpose. Mr Jeremy, both you and I know that your payment for making the first truly accurate clock will be the opportunity to make the first truly accurate clock, yes?' He smiled nervously. 'It would be... wonderful, if it could be done,' he said. 'Really, it would... be the end of clockmaking.'

'Yes,' said Lady LeJean. 'No one would ever have to make a clock again.' Tick This desk is neat.

There is a pile of books on it, and a ruler. There is also, at the moment, a clock made out of cardboard. Miss picked it up. The other teachers in the school were known as Stephanie and Joan and so on, but to her class she was very strictly Miss Susan. 'Strict', in fact, was a word that seemed to cover everything about Miss Susan and, in the classroom, she insisted on the Miss in the same way that a king insists upon Your Majesty, and for pretty much the same reason. Miss Susan wore black, which the headmistress disapproved of but could do nothing about because black was, well, a respectable colour. She was young, but with an indefinable air of age about her. She wore her hair, which was blond-white with one black streak, in a tight bun. The headmistress disapproved of that, too - it suggested an Archaic Image of Teaching, she said, with the assurance of someone who could pronounce a capital letter. But she didn't ever dare disapprove of the way Miss Susan moved, because Miss Susan moved like a tiger. It was in fact always very hard to disapprove of Miss Susan in her presence, because if you did she gave you a Look. It was not in any waya threatening look. It was cool and calm. You just didn't want to see it again. The Look worked in the classroom, too. Take homework, another Archaic Practice the headmistress was ineffectually Against. No dog ever ate the homework of one of Miss Susan's students, because there was something about Miss Susan that went home with them; instead the dog brought them a pen and watched imploringly while they finished it. Miss Susan seemed to have an unerring instinct for spotting laziness and effort, too. Contrary to the headmistress's instructions, Miss Susan did not let the children do what they liked. She let them do what she liked. It had turned out to be a lot more interesting for everyone. Miss Susan held up the cardboard clock and said: 'Who can tell me what this is?' A forest of hands shot up. 'Yes, Miranda?'

'It's a clock, miss.' Miss Susan smiled, carefully avoided the hand that was being waved by a boy called Vincent, who was also making frantically keen 'ooo, ooo, ooo' noises, and chose the one behind him. 'Nearly right,' she said. 'Yes, Samuel?'

'It's all cardboard made to look like a clock,' said the boy. 'Correct. Always see what's really there. And I'm supposed to teach you to tell the time with this.' Miss Susan gave it a sneer and tossed it away. 'Shall we try a different way?' she said, and snapped her fingers. 'Yes!' the class chorused, and then it went 'Aah!' as the walls, floor and ceiling dropped away and the desks hovered high over the city.

A few feet away was the huge cracked face of the tower clock of Unseen University. The children nudged one another excitedly. The fact that their boots were over three hundred feet of fresh air didn't seem to bother them. Oddly, too, they did not seem surprised. This was just an interesting thing. They acted like connoisseurs who had seen other interesting things. You did, when you were in Miss Susan's class. 'Now, Melanie,' said Miss Susan, as a pigeon landed on her desk. 'The big hand is on the twelve and the enormous hand is nearly on the ten, so it's...' Vincent's hand shot up. 'Ooo, miss, ooo, ooo ...'

'Nearly twelve o'clock,' Melanie managed. 'Well done. But here. . .' The air blurred. Now the desks, still in perfect formation, were firmly on the cobbles of a plaza in a different city. So was most of the classroom. There were the cupboards, and the Nature Table, and the blackboard. But the walls still lagged behind. No one in the plaza paid the visitors any attention but, oddly, no one tried to walk into them either. The air was warmer, and smelled of sea and swamp. 'Anyone know where this is?' said Miss Susan. 'Ooo, me, miss, ooo, ooo ...' Vincent could only stretch his body taller if his feet left the ground. 'How about you, Penelope?' said Miss Susan. 'Oh, miss,' said a deflated Vincent. Penelope, who was beautiful, docile and frankly dim, looked around at the thronged square and the whitewashed, awning-hung buildings with an expression close to panic. 'We came here in geography last week,' said Miss Susan. 'City surrounded by swamps. On the Vieux river. Famous cookery. Lots of seafood... ?' Penelope's exquisite brow creased. The pigeon on Miss Susan's desk fluttered down and joined the pigeon flock prospecting for scraps among the flagstones, cooing gently to the others in pidgin pigeon. Aware that a lot could happen while people waited for Penelope to complete a thought process, Miss Susan waved at a clock on a shop across the square and said: 'And who can tell me the time here in Genua, please?'

'Ooo, miss, miss, ooo ...' A boy called Gordon cautiously admitted that it might be three o'clock, to the audible disappointment of the inflatable Vincent.

'That's right,' said Miss Susan. 'Can anyone tell me why it's three o'clock in Genua while it's twelve o'clock in Ankh-Morpork?' There was no avoiding it this time. If Vincent's hand had gone up any faster it would have fried by air friction. 'Yes, Vincent?'

'Ooo miss speed of light miss it goes at six hundred miles an hour and at the moment the sun's rising on the Rim near Genua so twelve o'clock takes three hours to get to us miss!' Miss Susan sighed. 'Very good, Vincent,' she said, and stood up. Every eye in the room watched her as she crossed over to the Stationery Cupboard. It seemed to have travelled with them and now, if there had been anyone to note such things, they might have seen faint lines in the air that denoted walls and windows and doors. And if they were intelligent observers, they'd have said: so ... this classroom is in some way still in Ankh-Morpork and also in Genua, is it? Is this a trick? Is this real? Is it imagination? or is it that, to this particular teacher, there is not much of a difference? The inside of the cupboard was also present, and it was in that shadowy, paper-smelling recess that she kept the stars. There were gold stars and silver stars. One gold star was worth three silver ones. The headmistress disapproved of these, as well. She said they encouraged Competitiveness. Miss Susan said that was the point, and the headmistress scuttled away before she got a Look. Silver stars weren't awarded frequently and gold stars happened less than once a fortnight, and were vied for accordingly. Right now Miss Susan selected a silver star. Pretty soon Vincent the Keen would have a galaxy of his very own. To give him his due, he was quite uninterested in which kind of star he got. Quantity, that was what he liked. Miss Susan had privately marked him down as Boy Most Likely to Be Killed One Day By His Wife. She walked back to her desk and laid the star, tantalizingly, in front of her. 'And an extra-special question,' she said, with a hint of malice. 'Does that mean it's “then” there when it's “now” here?' The hand slowed halfway in its rise. 'Ooo ...' Vincent began, and then stopped. 'Doesn't make sense, miss...'