When you read/see a Shakespeare play (or any other old literature) and start to talk like the characters (i.e. vocabulary, mannerisms, rhythm, rhyme, iambic pentameter, etc.) on a regular basis.
Guy 1: She doth teach the torches to burn bright!
Guy 2: Dude... what!?
Guy 1: I said that chicks hot, man!
Guy 2: Oh, what was all that you were saying?
Guy 1: Sorry, dude. I've been speaking Shakespeare ever since we read Romeo and Juliet in class last week.