In Act 3, Scene 2, after the players’ performance causes Claudius to storm off, Hamlet has a conversation with his friends Rosencrantz and Guildenstern. They have been sent by Hamlet’s mother, who is concerned by his behavior. Hamlet seems immediately hostile to the envoy and doesn’t give his friends a straight answer. Then, when the players reenter with their instruments, he picks up a flute and asks them if they will play it. Guildenstern protests that he doesn’t know how, and the simile Hamlet sets forth in response ultimately hints at the source of his anger and the tension in the scene. He says:
It is as easy as lying. Govern these ventages
with your fingers and thumb, give it breath with
your mouth and it will discourse most eloquent
music. Look you, these are the stops.
In this passage, Hamlet seems to be making fun of his friends. His use of a simile is part of what creates this sense. Playing the flute is "as easy as lying" because all it takes to make a sound is for the player to fill the instrument with air—or, to put it another way, to fill it with nothingness. The implication here is that the things his friends have told him are lacking in substance and truth. Hamlet’s employment of the simile is a way to make fun of his friends, to make their stupidity apparent without taunting them outright.