Cheryl's Birthday: the logic puzzle that broke the internet in 2015
Cheryl tells Albert and Bernard separately that her birthday is one of: May 15, 16, 19; June 17, 18; July 14, 16; August 14, 15, 17. She tells Albert only the month and 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, 'Then I know too.' When is Cheryl's birthday?
Reveal the answer
July 16. Albert's first statement rules out May and June, since May 19 and June 18 are days unique enough that Bernard might already know from them alone — leaving July 14, 16 and August 14, 15, 17. Bernard's statement rules out the 14th, since it's ambiguous between July and August — leaving July 16, August 15 and August 17. Albert's final statement means his month must leave only one option, which only works for July. The puzzle appeared in the 2015 Singapore and Asian Schools Math Olympiad for 14-year-olds and went viral worldwide.