Library›Chance & Paradoxes›One card

A LITTLE SOMETHING WORTH KNOWING

A small discovery. Yours to pass on.

Math & LogicOne small ahaProbability or paradox

This birthday-sharing puzzle reaches just over a 50% match probability with 23 people under the usual equal-day assumptions.

Think it. Then flip it. ahamoraTap to reveal Back to the cue
Math & LogicOne small aha
Your new ahaThe Birthday Paradox

The Birthday Paradox: Shows how quickly shared birthdays become likely in a group. Source: Birthday problem ↗

ahamoraReference ↗

Keep the thread going

One answer is a beginning.

Share a little curiosity, or follow it into the full collection.

Flip the full deck →

More from Chance & Paradoxes

A few more worth knowing.

  1. 10In this statistical paradox, a trend in separate groups reverses when the groups are combined.RECALL
    Simpson's Paradox

    Simpson's paradox shows a data trend can reverse once separate groups are combined, a caution for statistics.

    Open this card ↗
  2. 11One sealed envelope contains twice the money in the other; this puzzle exposes faulty reasoning that seems to favour switching whichever one you chose.RECALL
    The Two Envelopes Problem

    The two envelopes problem seems to always favor switching, exposing a flaw in naive expected-value reasoning.

    Open this card ↗
  3. 12In this two-player game-theory scenario, individually rational betrayal can leave both worse off than cooperation.RECALL
    The Prisoner's Dilemma

    The prisoner's dilemma shows individually rational choices can leave both players worse off, a founding game theory scenario.

    Open this card ↗