Ten possible dates, two facts whispered separately — can you still find the birthday?
Albert is told only the month of Cheryl's birthday, Bernard only the day. Albert says: 'I don't know when her birthday is, but I know Bernard doesn't know either.' Bernard replies: 'At first I didn't know, but now I do.' Albert then says: 'Well, now I know too.' Given the ten possible dates, what is Cheryl's birthday?
Reveal the answer
16 July. Albert's first statement rules out any month containing a day unique enough for Bernard to already know the answer; Bernard's reply narrows the field to dates left unique after that; Albert's final claim narrows it to exactly one date. The question appeared in the 2015 Singapore and Asian Schools Math Olympiad, aimed at 14-year-olds, and went viral online for stumping adults.
— Joseph Yeo Boon Wooi / SASMO, Singapore and Asian Schools Math Olympiad — 2015