Margen
How many people do you need in a room for it to be more likely than not that two of them share a birthday? Intuition shouts hundreds. The correct answer is 23.
The trick is that we are not looking for someone to match you, but for any two people to match each other. With 23 people there are 253 possible pairs, and each pair is a chance of a match. Added together, they pass 50 %. With 57 people, the probability already brushes 99 %.
The lesson goes beyond parties: coincidences are far more common than our brain reckons. The effect is so powerful that cryptographers use it to attack security systems: they call it, quite literally, the "birthday attack".
Source: Scientific American