On subway systems and tall buildings: Isn’t it strange to think of how many people are living lives below your feet?

On the way sometimes, when you’re sad and a stranger is nice to you, it makes you want to cry: I read a book recently in which it was hypothesized that this happens because you grow depressed that only a stranger is willing to be kind to you, leading to you feel even more alone. But that’s not the right answer, is it? I don’t think it’s even close.