Going Postal

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 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 in 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."