"You lied to Mum and Dad, didn't you?" Kitty asked, skipping a little bit in order to keep up with the Brigadier's long strides. "About me studying maths for the government."
The Brigadier stared down at her. Kitty stared back; she was only thirteen (and a half - it was the sort of stare that made her add the "and a half" in sheer stubbornness), but she could walk through walls, and the Brigadier knew she could walk through walls. She didn't have to be scared of him... she told herself.
"No, I did not," he replied crisply as they reached the truck. "I told them the exact truth. You will be continuing your studies in advanced maths, and you will be working for an agency of the British government." He paused and helped her scramble into the truck. He had turned on the ignition before he spoke again. "However, if you want... you will also receive training as part of an elite task force: Earth's first line of defence against alien invasion. How does that sound?"
For a second, Kitty couldn't even talk. She'd suspected something like this, of course, ever since the Brigadier turned up at her house - it'd have to be this or locking her up forever, since he'd let her leave the subway (through the wall) after seeing those Yeti monsters without even signing a nondisclosure agreement - but actually hearing it was too much.
That state of affairs only lasted a second. Kitty was, after all, thirteen years old.