Thank you to Elena Atroshchenko for proposing this teaser
Albert and Bernard have just met with Cheryl. They want to know when her birthday is. Cheryl gave them ten possible dates: May 15, May 16, May 19, June 17, June 18, July 14, July 16, August 14, August 15 and 17 August. Then Cheryl told Albert the month of her birthday, and told Bernard – the day.
After that, the following dialogue took place.
Albert: I do not know when Cheryl’s birthday is, but I know that Bernard does not know either.
Bernard: At first I did not know when Cheryl’s birthday is, but I know now.
Albert: Now I also know when Cheryl’s birthday is.
When is Cheryl’s birthday?