In a striking analogy Hamlet accuses Guildenstern of playing him like what?

Hamlet accuses Guildenstern of playing him like a "pipe." He says, "Do you think I am easier to be played on than a pipe? Call me what instrument you will, though you can fret me, you cannot play upon me." Hamlet likens himself to a pipe because he feels that he is easily seen and heard by others, but that he cannot express his own thoughts or feelings. He is also saying that Guildenstern is trying to control him, but that he will not let him.