Just post a quote, from a famous speech, literature, movie/tv show, song, etc.
If someone knows what it's from, they identify where it's from (author, actor, title, etc) & then post their own quote.
As example:
poster #1: To be, or not to be?
poster #2: From the play Hamlet.
Get thee to a nunnery!
...And so on.
Stick to mass-circulated references, avoid things that only only locals in your area will get or are too obscure (such as, "My bud Chet said it in Home Ec last week.")