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.’

‘And that was the end of it, sir,’ said Mr Groat, as they left the room. ‘Actually, I heard where the wizards were saying that the universe was destroyed all in one go but instantly came back all in one go. They said they could tell by lookin’, sir. So that was okay and it let old Rumbelow off’f the hook, on account it’s hard to discipline a man under Post Office Regulations for destroying the universe all in one go. Mind you - hah - there’ve been postmasters that would have given it a try. But it knocked the stuffing out of us, sir. It was all downhill after that. The men had lost heart. It broke us, to tell you the truth.’

‘Look,’ said Moist, ‘the letters we’ve just given the lads, they’re not from some other dimension or—’

‘Don’t worry, I checked ‘em last night,’ said Groat. ‘They’re just old. Mostly you can tell by the stamp. I’m good at telling which ones are prop’ly ours, sir. Had years to learn. It’s a skill, sir.’

‘Could you teach other people?’

‘I dare say, yes,’ said Groat.

‘Mr Groat, the letters have talked to me,’ Moist burst out.

To his surprise, the old man grabbed his hand and shook it. ‘Well done, sir!’ he said, tears rising in his eyes. ‘I said it’s a skill, didn’t I? Listen to the whispers, that’s half the trick! They’re alive, sir, alive . Not like people, but like… ships are alive, sir. I’ll swear, all them letters pressed together in here, all the… the passion of ‘em, sir, why, I do think this place has got something like a soul, sir, indeed I do… ’

The tears coursed down Groat’s cheeks. It’s madness, of course, thought Moist. But now I’ve got it, too.

‘Ah, I can see it in your eyes, sir, yes I can!’ said Groat, grinning wetly. ‘The Post Office has found you! It’s enfolded you, sir, yes it has. You’ll never leave it, sir. There’s families that’ve worked here for hundreds and hundreds of years, sir. Once the postal service puts its stamp on you, sir, there’s no turning back… ’

Moist disentangled his hand as tactfully as he could.

‘Yes,’ he said. ‘Do tell me about stamps.’

Thump.

Moist looked down at the piece of paper. Smudgy red letters, chipped and worn, spelled out: ‘Ankh-Morpork Post Office.’

‘That’s right, sir,’ said Groat, waving the heavy metal and wood stamper in the air. ‘I bang the stamp on the ink pad here, then bang it, sir, bang it on the letter. There! See? Done it again. Same every time. Stamped.’

‘And this is worth a penny?’ said Moist. ‘Good grief, man, a kid could forge this with half a potato!’

‘That was always a bit of a problem, sir, yes,’ said Groat.

‘Why does a postman have to stamp the letters, anyway?’ said Moist. ‘Why don’t we just sell people a stamp?’

‘But they’d pay a penny and then go on stamping for ever, sir,’ said Groat, reasonably.

In the machinery of the universe, the wheels of inevitability clicked into position…

‘Well, then,’ said Moist, staring thoughtfully at the paper, ‘how about… how about a stamp you can use only once?’

‘You mean, like, not much ink?’ said Groat. His brow wrinkled, causing his toupee to slip sideways.

‘I mean… if you stamped the stamper lots of times on paper, then cut out all the stampings… ’ Moist stared at an inner vision, if only to avoid the sight of the toupee slowly crawling back. ‘The rate for delivery anywhere in the city is a penny, isn’t it?’

‘Except for the Shades, sir. That’s five pence ‘cos of the armed guard,’ said Groat.

‘Right. O-kay. I think I might have something here… ’ Moist looked up at Mr Pump, who was smouldering in the corner of the office. ‘Mr Pump, would you be so good as to go along to the Goat and Spirit Level over at Hen-and-Chickens and ask the publican for “Mr Robinson’s box”, please? He may want a dollar. And while you’re over there, there’s a printing shop over that way, Teemer and Spools. Leave a message to say that the Postmaster General wishes to discuss a very large order.’