'Thank you. Er . . . right.' Moist wasn't sure what he was inspecting, but he did his best. Wrinkled face after wrinkled face stared back at him. The medals, he realized, weren't all for military service. The Post Office had medals of its own. One was a golden dog's head, worn by a little man with a face like a packet of weasels. 'What's this, er . . .' he began. 'Senior Postman George Aggy, sir. The badge? Fifteen bites and still standin', sir!' said the man proudly. 'Well, that is a . . . a . . . a lot of bites, isn't it . . .'

'Ah, but I foxed 'em after number nine, sir, and got meself a tin leg, sir!'

'You lost your leg?' said Moist, horrified. 'No, sir. Bought a bit of ol' armour, didn't I?' said the wizened man, grinning artfully. 'Does m'heart good to hear their teeth squeaking, sir!'

'Aggy, Aggy . . .' Moist mused, and then memory sparked. 'Weren't you—'

'I'm the Worshipful Master, sir,' said Aggy. 'I hope you won't take last night the wrong way, sir. We all used to be like young Tolliver, sir, but we gave up hope, sir. No hard feelings?'

'No, no,' said Moist, rubbing the back of his head. 'And I'd like to add my own message of congratulations as chairman of the Ankh-Morpork Order of Postal Workers Benevolent and Friendly Society,' Aggy went on. 'Er . . . thank you,' said Moist. 'And who are they, exactly?'

'That was us last night, sir,' said Aggy, beaming. 'But I thought you were a secret society!'

'Not secret, sir. Not exactly secret. More . . . ignored, you might say. These days it's just about pensions and making sure your ol' mates get a proper funeral when they're Returned to Sender, really.'

'Well done,' said Moist vaguely, which seemed to cover everything. He stood back, and cleared his throat. 'Gentlemen, this is it. If we want the Post Office back in business, we must start by delivering the old mail. It is a sacred trust. The mail gets through. It may take fifty years, but we get there in the end. You know your walks. Take it steady. Remember, if you can't deliver it, if the house has gone . . . well, it comes back here and we'll put it into the Dead Letter office and at least we'll have tried. We just want people to know the Post Office is back again, understand?' A postman raised a hand. 'Yes?' Moist's skill at remembering names was better than his skill at remembering anything else about last night. 'Senior Postman Thompson, isn't it?'

'Yes, sir! So what do we do when people give us letters, sir?' Moist's brow wrinkled. 'Sorry? I thought you deliver the mail, don't you?'

'No, Bill's right, sir,' said Groat. 'What do we do if people give us new mail?'

'Er . . . what did you use to do?' said Moist. The postmen looked at one another. 'Get one penny off 'em for the stamping, bring it back here to be stamped with the official stamp,' said Groat promptly. 'Then it gets sorted and delivered.'

'So . . . people have to wait until they see a postman? That seems rather—'

'Oh, in the old days there was dozens of smaller offices, see?' Groat added. 'But when it all started going bad we lost 'em.'

'Well, let's get the mail moving again and we can work things out as we go along,' said Moist. 'I'm sure ideas will occur. And now, Mr Groat, you have a secret to share . . .'

Groat's key ring jingled as he led Moist through the Post Office's cellars and eventually to a metal door. Moist noted a length of black and yellow rope on the floor: the Watch had been here, too. The door clicked open. There was a blue glow inside, just faint enough to be annoying, leave purple shadows on the edge of vision and make the eyes water. 'Voil-ah,' said Groat. 'It's a . . . is it some kind of theatre organ?' said Moist. It was hard to see the outlines of the machine in the middle of the floor, but it stood there with all the charm of a torturer's rack. The blue glow was coming from somewhere in the middle of it. Moist's eyes were streaming already. 'Good try, sir! Actually it is the Sorting Engine,' said Groat. 'It's the curse of the Post Office, sir. It had imps in it for the actual reading of the envelopes, but they all evaporated years ago. Just as well, too.' Moist's gaze took in the wire racks that occupied a whole wall of the big room. It also found the chalk outlines on the floor. The chalk glowed in the strange light. The outlines were quite small. One of them had five fingers. 'Industrial accident,' he muttered. 'All right, Mr Groat. Tell me.'

'Don't go near the glow, sir,' said Groat. 'That's what I said to Mr Whobblebury. But he snuck down here all by hisself, later on. Oh, dear, sir, it was poor young Stanley that went and found him, sir, after he saw poor little Tiddles dragging something along the passage. A scene of carnage met his eyes. You just can't imagine what it was like in here, sir.'

'I think I can,' said Moist. 'I doubt if you can, sir.'

'I can, really.'

'I'm sure you can't, sir.'

'I can! All right?' shouted Moist. 'Do you think I can't see all those little chalk outlines? Now can we get on with it before I throw up?'

'Er . . . right you are, sir,' said Groat. 'Ever heard of Bloody Stupid Johnson? Quite famous in this city.'

'Didn't he build things? Wasn't there always something wrong with them? I'm sure I read something about him—'

'That's the man, sir. He built all kinds of things, but, sad to say, there was always some major flaw.' In Moist's brain, a memory kicked a neuron. 'Wasn't he the man who specified quicksand as a building material because he wanted a house finished fast?' he said. 'That's right, sir. Usually the major flaw was that the designer was Bloody Stupid Johnson. Flaw, you might say, was part of the whole thing. Actually, to be fair, a lot of the things he designed worked quite well, it was just that they didn't do the job they were supposed to. This thing, sir, did indeed begin life as an organ, but it ended up as a machine for sorting letters. The idea was that you tipped the mail sack in that hopper, and the letters were speedily sorted into those racks. Postmaster Cowerby meant well, they say. He was a stickler for speed and efficiency, that man. My grandad told me the Post Office spent a fortune on getting it to work.'

'And lost their money, eh?' said Moist. 'Oh no, sir. It worked. Oh yes, it worked very well. So well that people went mad, come the finish.'

'Let me guess,' said Moist. 'The postmen had to work too hard?'

'Oh, postmen always work too hard, sir,' said Groat, without blinking. 'No, what got people

worried was finding letters in the sorting tray a year before they were due to be written.' There was a silence. In that silence, Moist tried out a variety of responses, from 'Pull the other one, it's got bells on' to 'That's impossible', and decided they all sounded stupid. Groat looked deadly serious. So instead he said: 'How?' The old postman pointed to the blue glow. 'Have a squint inside, sir. You can just see it. Don't get right above it, whatever you do.' Moist moved a little closer to the machine and peered into the machinery. He could just make out, at the heart of the glow, a little wheel. It was turning, slowly. 'I was raised in the Post Office,' said Groat, behind him. 'Born in the sorting room, weighed on the official scales. Learned to read from envelopes, learned figuring from old ledgers, learned jography from looking at the maps of the city and history from the old men. Better than any school. Better than any school, sir. But never learned jommetry, sir. Bit of a hole in my understanding, all that stuff about angles and suchlike. But this, sir, is all about pie.'

'Like in food?' said Moist, drawing back from the sinister glow. 'No, no, sir. Pie like in jommetry.'

'Oh, you mean pi the number you get when . . .' Moist paused. He was erratically good at maths, which is to say he could calculate odds and currency very, very fast. There had been a geometry section in his book at school, but he'd never seen the point. He tried, anyway. 'It's all to do with . . . it's the number you get when the radius of a circle . . . no, the length of the rim of a wheel is three and a bit times the . . . er . . .'

'Something like that, sir, probably, something like that,' said Groat. 'Three and a bit, that's the ticket. Only Bloody Stupid Johnson said that was untidy, so he designed a wheel where the pie was exactly three. And that's it, in there.'

'But that's impossible!' said Moist. 'You can't do that! Pi is like . . . built in! You can't change it. You'd have to change the universe!'

'Yes, sir. They tell me that's what happened,' said Groat calmly. 'I'll do the party trick now. Stand back, sir.' Groat wandered out into the other cellars and came back with a length of wood. 'Stand further back, sir,' he suggested, and tossed the piece of wood on top of the machine. The noise wasn't loud. It was a sort of 'clop'. It seemed to Moist that something happened to the wood when it went over the light. There was a suggestion of curvature— Several pieces of timber clattered on to the floor, along with a shower of splinters. 'They had a wizard in to look at it,' said Groat. 'He said the machine twists just a little bit of the universe so pie could be three, sir, but it plays hob with anything you put too near it. The bits that go missing get lost in the . . . space-time-continuememememem, sir. But it doesn't happen to the letters because of the way they travel through the machine, you see. That's the long and short of it, sir. Some letters came out of that machine fifty years before they were posted!'

'Why didn't you switch it off?'

'Couldn't, sir. It kept on going like a siphon. Anyway, the wizard said if we did that terrible things might happen! 'cos of, er, quantum, I think.'

'Well, then, you could just stop feeding it mail, couldn't you?'

'Ah, well, sir, there it is,' said Groat, scratching his beard. 'You have positioned your digit right on the nub or crux, sir. We should've done that, sir, we should've, but we tried to make it work for us, you see. Oh, the management had schemes, sir. How about delivering a letter in Dolly Sisters thirty seconds after it had been posted in the city centre, eh? Of course, it wouldn't be polite to deliver mail before we'd actually got it, sir, but it could be a close run thing, eh? We were good, so we tried to be better . . .' And, somehow, it was all familiar . . .

Moist listened glumly. Time travel was only a kind of magic, after all. That's why it always went wrong. That's why there were postmen, with real feet. That's why the clacks was a string of expensive towers. Come to that, it was why farmers grew crops and fishermen trawled nets. Oh, you could do it all by magic, you certainly could. You could wave a wand and get twinkly stars and a fresh-baked loaf. You could make fish jump out of the sea ready cooked. And then, somewhere, somehow, magic would present its bill, which was always more than you could afford. That's why it was left to wizards, who knew how to handle it safely. Not doing any magic at all was the chief task of wizards - not 'not doing magic' because they couldn't do magic, but not doing magic when they could do and didn't. Any ignorant fool can fail to turn someone else into a frog. You have to be clever to refrain from doing it when you know how easy it is. There were places in the world commemorating those times when wizards hadn't been quite as clever as that, and on many of them the grass would never grow again. Anyway, there was a sense of inevitability about the whole business. People wanted to be fooled. They really believed that you found gold nuggets lying on the ground, that this time you could find the Lady, that just for once the glass ring might be real diamond. Words spilled out of Mr Groat like stashed mail from a crack in the wall. Sometimes the machine had produced a thousand copies of the same letter, or filled the room with letters from next Tuesday, next month, next year. Sometimes they were letters that hadn't been written, or might have been written, or were meant to have been written, or letters which people had once sworn that they had written and hadn't really, but which nevertheless had a shadowy existence in some strange invisible letter world and were made real by the machine. If, somewhere, any possible world can exist, then somewhere there is any letter that could possibly be written. Somewhere, all those cheques really are in the post. They poured out - letters from the present day which turned out not to be from this present day, but ones that might have happened if only some small detail had been changed in the past. It didn't matter that the machine had been switched off, the wizards said. It existed in plenty of other presents and so worked here owing to . . . a lengthy sentence which the postmen didn't understand but had words like 'portal', 'multidimensional' and 'quantum' in it, quantum being in it twice. They didn't understand, but they had to do something. No one could deliver all that mail. And so the rooms began to fill up . . . The wizards from Unseen University had been jolly interested in the problem, like doctors being really fascinated by some new virulent disease; the patient appreciates all the interest, but would very much prefer it if they either came up with a cure or stopped prodding. The machine couldn't be stopped and certainly shouldn't be destroyed, the wizards said. Destroying the machine might well cause this universe to stop existing, instantly. On the other hand, the Post Office was filling up, so one day Chief Postal Inspector Rumbelow had gone into the room with a crowbar, had ordered all the wizards out, and belted the machine until things stopped whirring. The letters ceased, at least. This came as a huge relief, but nevertheless the Post Office had its regulations and so the Chief Postal Inspector was brought before Postmaster Cowerby and asked why he had decided to risk destroying the whole universe in one go. According to Post Office legend, Mr Rumbelow had replied: 'Firstly, sir, I reasoned that if I destroyed the universe all in one go no one would know; secondly, when I walloped the thing the first time the wizards ran away, so I surmised that unless they had another universe to run to they weren't really certain; and lastly, sir, the bloody thing was getting on my nerves. Never could stand machinery, sir.'