Lady Macbeth tells her husband to "look like the innocent flower/ But be the serpent under it." Explain what she means.

Essentially, this line is Lady Macbeth's warning to her
husband in how to engage in immorality to get what they both want.  She says this to him
in Act I, sc. 5, when he tells her that Duncan is coming.  Lady Macbeth recognizes this
as the perfect opportunity for Macbeth to kill Duncan and seize power.   In order to
become King, Lady Macbeth realizes that Macbeth must kill Duncan.  She also understands
that he lacks the vocabulary and full understanding to do so.  As a warrior and a
fighter on the battlefield, murder is done in name of King and country and there is
little duplicity involved.  The slaughter is understood.  Lady Macbeth shrewdly realizes
that her husband might need some level of guidance in how to murder for personal gain. 
It is in this where she advises him to "beguile time," and put on pretenses as if he is
a gracious host, and devoid of any malicious intent.  The "serpent under it" is how she
believes Macbeth will best understand what needs to be done in how the murder should be
executed.  In this line, Lady Macbeth's initial deviousness is evident, something that
will change over the course of the play.  At the same time, Macbeth's overall innocence
is also evident, something that will also change over the course of the
play.