Put twenty-three people in a room and the odds are slightly better than even that two of them share a birthday. It sounds wrong because everyone checks it the same way, against themselves: is anyone here born on my day? For that, you would need about 253 people before the odds tipped past half. And 253 happens to be exactly how many pairs twenty-three people make, each one a small separate chance at a match. The surprise was never in the people. A room is mostly made of the gaps between them, and there are far more of those than there are of us.