If you haven't heard the riddle by now, no need to worry! The riddle is: There are 30 cows and 28 chickens. How many didn't?

This article was originally published on mentalfloss.com as Can You Solve This Old-Timey Riddle? #9.

While some groupings might seem obvious at first glance, only one correct answer exists for each category. The challenge lies in recognizing subtle patterns, whether they pertain to book titles, pop ...

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As these were pre-literate societies, one can only guess at the motive ... now argues that a global epidemic may explain the genetic riddle. The theory is unconvincing. The Black Death that ...