"I know," she answers - she can hear his single heart, after all. "And that won't kill them," she adds. "Actually, I have no idea how to kill them permanently. They can regenerate, they're over a thousand years old or something." Hey, better Peter hears about it from her, right? If it becomes an issue. "The Doctor - that's your double's name - won't harm you unless you hurt someone yourself. He's got a bit of a hero complex."