"“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 the wheel is three and a bit times the…er…” “Something like that, sir, probably, something like that,” said Grote. “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."