Birthday paradox 

In probability theory, Birthday paradox is the phenomenon concerning the probability that, in a set of n randomly chosen people, at least two will share the same birthday. The birthday paradox is the counterintuitive fact that only 23 people are needed for that probability to exceed 50%. The birthday paradox is a veridical paradox, i.e. a paradox that produces counter-intuitive results, but is demonstrated to be true nonetheless.

I always thought it was super rare for two people to share a birthday, but the birthday paradox says it only takes 23 people to make it likely!

Birthday paradox by Emotional Cruiser September 26, 2025