Maybe this has made the rounds here but I hadn't seen it. A friend sent this via email, which I will paste.

Albert and Bernard just became friends with Cheryl and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates:

May 15 16 19

June 17 18

July 14 16

August 14 15 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.

(So Albert knows only the month and Bernard knows only the date.)

Then Albert and Bernard have the following conversation:

Albert: I don't know when Cheryl's birthday is, but I know that Bernard doesn't know either.

Bernard: At first I didn't know when Cheryl's birthday is, but I know now.

Albert: Then I also know when Cheryl's birthday is.

So when is Cheryl's birthday?"

Albert and Bernard just became friends with Cheryl and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates:

May 15 16 19

June 17 18

July 14 16

August 14 15 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.

(So Albert knows only the month and Bernard knows only the date.)

Then Albert and Bernard have the following conversation:

Albert: I don't know when Cheryl's birthday is, but I know that Bernard doesn't know either.

Bernard: At first I didn't know when Cheryl's birthday is, but I know now.

Albert: Then I also know when Cheryl's birthday is.

So when is Cheryl's birthday?"