Fact
Twenty-three people give you a better than even chance of a shared birthday
It feels like it should take hundreds. It takes a small classroom.
The question is not how many people share your birthday, but whether any pair shares one – and the number of pairs grows quadratically. With 23 people there are 253 pairs, pushing the probability just above 50 per cent. At 70 people it is well over 99 per cent.