Hogfather - Cone

Susan advanced on the heap, moving warily. They were small teeth. Children's teeth. Whoever had piled them up hadn't been very careful about it, either. A few had been scattered across the floor. She knew because she trod on one, and the slippery little crunching sound made her desperate not to tread on any more. Whoever had piled them up had presumably been the one who'd drawn the chalk marks around the obscene heap.

'There're so many,' whispered Bilious.

'At least twenty million, given the size of the average milk tooth,' said Susan. She was shocked to find that it came almost automatically.

'How can you possibly know that?'

'Volume of a cone,' said Susan. 'Pi times the square of the radius times the height divided by three. I bet Miss Butts never thought it'd come in handy in a place like this.'

'That's amazing. You did it in your head?’

***

Hogfather - p274