Oh man, I'm getting old.
I have a total needed for this puzzle:
40 minutes time
2 Playmobil figures, Bernard & Albert should represent
10 Post It with the months and days
And the bad thing about it
The puzzle is for 14 year old students
"I don't know when Cheryl's birthday is, but I know that Bernard doesn't know it too." Rules out May and June, as Bernard could possibly know, if he was told 19 (May) or 18 (June).
"At first I didn't know when Cheryl's birthday is, but I know now." Rules out July 14 and Aug 14, as if it were one of these, he still could not know.
"Then I also know when Cheryl's birthday is." Must be July 16, if he knew it was in "August," he still could not know the date.