"Vampires were the next obvious one after dragons because their lifespans are obviously determined by magic," she says. "I think I'll have to invent something different and more general for everyone else, because I don't think lifespan is that simple for most species. And I think it might be hard to tell if something I try has worked, depending on what it is exactly. When there's magic directly involved then I can just see what's happening, but plain biology is much trickier, I'm not at all sure how to teach ialdae to see that."