On family road trips when I was a kid, I remember looking at the flow of cars in the opposing lanes, and thinking about just how many people there were in the world: We'd pass another car every second or so with at least one person in it, for hours and hours. It was a never-ending parade of humanity, and with only a handful of exceptions, people I would never, ever see again. The mind can't grasp those kinds of numbers.

And I'm old, so there are, like, almost twice as many people now.