"I never learned jommetty, 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 math, 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 untldy, 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 slop. 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 onto 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 pi 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 oh er, quantum, l think."

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

"Ah, well, sir, there it is," said Croat, snatching his beard. "You have positioned your digit right on the nub, or crust, sir. Nyle 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 center, 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 grimly. Time travel was only a kind of magic, after all. That's why it always went wrong.

That's why we're postmen, with real feet. ... 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 already cooked. And then, somewhere, somehow, magic would present its bill, which was always more than you could afford.