She flipped through the pages of the top booklet from the pile on her desk. It was the Year Seven version, so they were adorned with little cartoon magicians and demons. She was using last half term’s set—she needed to give these back to the class, actually, she’d have to look up where their form room was and drop them off. The workbook she was reviewing had been carefully, elaborately doodled on. Every cartoon demon had been given a little blue-ballpoint hat. Several of the fill-in-the-gap sections had been left blank. Hmm—so someone’s attention had been wandering badly, and Walden hadn’t caught it in the lesson. She needed to build in a little more roaming-the-room glancing-over-shoulders time for herself. Where could it go?

It took nearly forty minutes—half of that spent staring into space, rather than working.

Then something in Walden’s head went click.

“No,” she said out loud. “Come on.”

Click, click, like the pneumatic cylinders behind the walnut panels of the thaumic engine sliding home one at a time. Like a train, finally pointed onto the correct track, starting to accelerate. Walden’s brain was a powerful, finely tuned, and very expensively educated problem-solving tool. Its weakness—the difference, if you liked, between being merely very intelligent and being brilliant—was a tendency to run safely on the rails of assumption.

It’s hardly likely. It’s not reasonable.

It couldn’t happen at Chetwood.

Walden flipped the Year Seven workbook closed. The blank back cover had been adorned with an impressively recognisable cartoon of herself, dressed up as a storybook magician, with pointy hat and long beard. Very good for a twelve-year-old. She smoothed it flat with both hands. Her fingernails were glowing gold. The air in her office was cold, suddenly, with a deep, brittle chill that owed nothing to January.

Ooooo, finally, crooned the Phoenix in her thoughts. Finally, finally.

Walden shivered. The demon that lived in her skin had leapt ahead of her and drawn its own conclusions, full of fiery joy. She asked: Did you know?

It did not understand. It knew its own nature, the hunter and the hunted. It knew the taste of danger and the calculus of risk and reward. It knew hunger, and it knew power. It was power. It liked the idea of a fight. It was not, Walden thought, a reliable witness.

“What if I’m wrong?” she asked the cool, antique silence. Then the answer, because it was obvious. “What if I’m not?”

The key to a good risk assessment was not How often does this happen. It was How bad could the consequences be?