Back when we'd been allowed outside sometimes, we'd learned that with every ten-degree drop, the cold became a new kind of thing. At ten it was brisk and energetic, like splashing your face with water, like having a dog lick your face. Zero is when it got bitter. At minus ten you felt the danger of it, like when you're very sick with a fever and your body feels like it's teetering somehow, even when you're lying down. Minus twenty we'd felt only a handful of times - when it got down that low every day they started locking the doors - but it was exciting, like when your fever really crests and you feel yourself splitting from your body.