← Paradoxes & riddles
1:46
Cheryl's Birthday
the logic puzzle that went viral
Cheryl tells Albert and Bernard that her birthday is one of 10 dates: May 15, 16 or 19; June 17 or 18; July 14 or 16; August 14, 15 or 17. She tells Albert only the month and Bernard only the day. Albert says: 'I don't know when it is, but I know Bernard doesn't know either.' Bernard says: 'At first I didn't know, but now I do.' Albert says: 'Then I know too.' When is Cheryl's birthday? The puzzle appeared in a Singapore math competition in 2015 and went viral.
Source: A classic puzzle, retold by INFINITE SUM and fact-checked against our curated notes
Want another one?
Type the name of a paradox or riddle you've heard of, or browse the rest of the archive.