Birthday

Get 23 people together and the odds two share a birthday already top 50%. With 365 days, you might expect far more, but the question is not "who matches me" — it is "do any two match." Among 23 there are 253 possible pairs. At 70 people, the probability hits 99.9%: the birthday paradox.

In Greek mythology, Artemis was the goddess of both the moon and childbirth. When a child was born, Greeks offered round cakes shaped like the moon to thank Artemis — believed to be the origin of eating cake on birthdays.