The Birthday Problem
You are at a party with 22 other people. What is more likely - that no two people have the same birth date (day and month) or that at least two people do have the same birth date?
This is called The Birthday Problem. Among 23 random people, the chance that two have the same birth date is 50.7%. Most people intuitively think that since there are 365 days in a year, the 50% threshold wouldn’t occur until around 182 or so.
Source: Wikipedia - The BIrthday Problem