"Look like th'innocent flower, but be the serpent under't." (Act 1 scene 6). Metaphor Shakespeare uses on Lady Macbeth while she is trying to convince Macbeth to commit the crime. Telling him to act soft and pure, like an innocent flower. But to be the serpent under it, the part of beautiful flowers such as roses that when you grab them for being beautiful, will hurt you. Just like Duncan will trust him, for Macbeth, to kill him later.