Why 23 people are enough for two to share a birthday

Put 23 people in a room and there's a better-than-even chance that two of them share a birthday. That sounds wrong, because 23 is tiny next to 365.

The catch is that nobody is being compared with you. Everyone is compared with everyone, and 23 people make 253 pairs. Each pair is a small chance of a match, and those small chances pile up fast. Fill a room yourself and watch the coincidences appear.