Doubtful it stood, 
As two spent swimmers that do cling together 
And choke their art. (1.2.7–9) 

Early in the play, the Captain uses this simile to compare the warring armies to a pair of exhausted swimmers hanging onto each other to try to stay above water, but instead threatening to pull each other under.   

Your face, my thane, is as a book where men 
May read strange matters.

Lady Macbeth uses this simile to tell her husband that his facial expressions betray his inner thoughts, likening him to a book that others can easily read.  

Look like th' innocent flower, 
But be the serpent under ’t.

In this simile, Lady Macbeth exhorts her husband to conceal his murderous intentions with innocent behavior, similar to a snake lurking beneath a harmless flower.  

The spring, the head, the fountain of your blood 
Is stopped; the very source of it is stopped.

Macbeth uses this metaphor to inform Donalbain and Malcolm of Duncan’s murder, characterizing their father as the fountain from which their lifeblood sprang and perhaps darkly hinting that their own lives are soon to be “stopped” as well.  

There the grown serpent lies. The worm that’s fled 
Hath nature that in time will venom breed; 
No teeth for th’ present. 

In this metaphor, Macbeth compares Banquo and his young son Fleance to two snakes, one a full-grown threat and the other a toothless baby snake who will one day become venomous like his father.  

He hath not touched you yet. I am young, but something 
You may deserve of him through me, and wisdom 
To offer up a weak, poor, innocent lamb 
T' appease an angry god.

In this metaphor, Malcolm compares himself to a sacrificial lamb and Macbeth to an angry god who would be pleased with his slaughter.  

He has no children. All my pretty ones? 
Did you say all? O hell-kite! All? 
What, all my pretty chickens and their dam 
At one fell swoop? 

After learning of his family’s massacre, Macduff uses this metaphor to compare Macbeth to a cruel kite (a type of hawk) who has swooped down and senselessly killed his entire brood of chicks and their mother all at once.  

Be this the whetstone of your sword. Let grief 
Convert to anger. Blunt not the heart, enrage it.

In this metaphor, Malcolm compares Macduff’s grief to a stone used for sharpening a sword, urging him to let grief sharpen, not dull, his desire for revenge.