Tech WondersPlay

Data & ProbabilityProbability

The Birthday Surprise

With just 23 people, a shared birthday is more likely than not

How many people does it take before two in a room probably share a birthday? Half of 365 sounds right. It isn't even close.

You'll find out

  • The real number, and why it feels wrong
  • Why mathematicians count the misses
  • Why it's almost never YOUR birthday

Why it matters

The same math sets how long your apps' IDs must be, and helps attackers break weak security far faster than expected.

About 3 min6 interactive slides · 5 questionsLesson 2 · Zone 1

Play this lesson

Free to play in your browser or on iPhone

More in this zone

  1. 1Graph TheorySix DegreesYou're about six people away from almost anyone
  2. 3Coding TheoryBroken QR Codes Still WorkRip out part of a QR code and it can still scan
  3. 4Map ProjectionsMercator's LieGreenland looks as big as Africa; Africa is 14 times bigger
  4. 5GeographyWhy Flights CurveThe shortest route looks bent on a flat map
  5. 6Natural Language ProcessingWhy AI Can't Count the R's in StrawberryAI reads chunks of words, not letters
  6. 7Internet & WebJourney of a ClickTap a link and your request circles the planet in under a second
  7. 8Visual PerceptionThe Blind SpotThere's a hole in your vision and your brain paints over it
  8. 9InfinityHilbert's HotelA full hotel with infinitely many rooms can always fit more guests