You know, the one about the probability of two people in a group having the same birthday. The probability exceeds 50% when there are 23 people in the group.

An explanation: 
*** http://www.mste.uiuc.edu/reese/birthday/intro.html

[And yet, it's not true, because in the RealWorld births are not evenly distributed.]

The limited variation in birthdate frequencies has a negligible effect on the overall result.

See also ProgrammerMathSkills and InevitableIllusions.
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CategoryMath