Oh, there's plenty of reasons. I just don't know which one.

Sometimes, the best answer is a more interesting question.

I express preference for a chronological sequence of events which precludes a violence.

For the first time in her life Granny wondered whether there might be something important in all these books people were setting store by these days, although she was opposed to books on strict moral grounds, since she had heard that many of them were written by dead people and therefore it stood to reason reading them would be as bad as necromancy. Among the many things in the infinitely varied universe with which Granny did not hold was talking to dead people, who by all accounts had enough troubles of their own.

Currently, it was leading him through a neighborhood that was on the downside of whatever curve you hoped you'd bought your property on the upside of. Graffiti and garbage were everywhere here. They were everywhere in the city, if it came to that, but elsewhere the garbage was better quality, and the graffiti was close to being correctly spelled. The whole area was waiting for something to happen, like a really bad fire.

Little fussy Otto, in his red-lined black opera cloak with pockets for all his gear, his shiny black shoes, his carefully cut widow's peak and, not least, his ridiculous accent that grew thicker or thinner depending on who he was talking to, did not look like a threat. He looked funny, a joke, a music-hall vampire. It had never previously occurred to Vimes that, just possibly, the joke was on other people.

A Thaum is the basic unit of magical strength. It has been universally established as the amount of magic needed to create one small white pigeon or three normal-sized billiard balls.