In the final scene of the first act of William Shakespeare's Hamlet, the title character, as part of his efforts to avenge the death of his father, decides that he will pretend to be insane.
In Act 2, Scene 1, we hear what is apparently the first manifestation of Hamlet's "madness." In this scene, set in her father Polonius' house, a highly agitated Ophelia rushes in and reports to her father that Hamlet had just appeared before her while she was sewing and was behaving in a very strange way. Ophelia reports that Hamlet was not dressed like a person usually would be, that he looked very pale, and he looked "As if he had been loosed out of hell."
Ophelia goes on to say that Hamlet did not say anything to her, but took her by the wrist, stared at her, sighed, and moved his head up and down three times. Ophelia and Polonius suspect that Hamlet is in love with her and that this is causing him to go insane: