Remember that blue dress from a few weeks ago? Or was it gold? There’s a new puzzle that has been leaving thousands of people scratching their heads across the globe, and this time it’s all about numerical logic.

Cheryl’s birthday challenge began as a test question at the Singapore and Asian Schools Math Olympiad in early April before being posted to Facebook by television host Kenneth Kong. In a short period of time, the problem became nearly as viral as #thedress. The Internet must love a good riddle because people from across a broad spectrum have weighed in with their answers. What began as an exercise for sophomore mathlete students was quickly picked up by professional mathematicians.

Without further ado, the problem is as follows: Albert and Bernard have just become friends with Cheryl and want to know when her birthday is. Instead of just telling them, Cheryl gives them a list of 10 possible dates. Her special day could fall on May 15, 16 or 19, Jun. 17, 18, Jul. 14, 16, Aug. 14, 15 or 17. Just to be especially confusing, Cheryl then separately tells Albert the month of her birthday and Bernard the day, after which they come together and discuss. Albert states that after all of this, he still does not know Cheryl’s birthday, but neither does Bernard. Bernard counters that while he did not know the birthday at first, now he does. Albert, in a moment of clarity, adds that he does too.

Social media responses to the question have ranged from serious answers to jibes at Cheryl’s strange behavior. If this is how she tells people about her birthday, how does she order pizza or ask someone on a date? One Twitter user joked that Bernard and Albert should have just added her on Facebook if they wanted to know her birth date. Of course, it must be assumed that all parties are being completely honest; if Bernard was lying about whether he knows the real date, the problem would become impossible to solve. The answer to the riddle has been posted by the New York Times, but reading it there is not as fun as figuring it out. If riddling out the date of Cheryl’s birth isn’t difficult enough, the popularity of the puzzle has inspired the Times to come up with an “afterparty” of additional word problems. Happy solving!

Activity: Try to solve the puzzle by thinking logically. Here’s a hint: write out the dates in a table format and look for patterns or differences between the numbers.

Jobs: Math teacher, logician.

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