In Shakespearean drama a foil is a character who is used as a contrast to another character. This contrast makes the particular qualities of each character stand out. In Romeo and Juliet Shakespeare uses the Nurse as a foil to Juliet and Mercutio as a foil to Romeo.
In Act I, Scene 3, we first meet Juliet and the Nurse. Juliet is a thirteen-year old girl who knows little of the world. In the preceding scene, her father, Lord Capulet says,
My child is yet a stranger in the world.
She hath not seen the change of fourteen years.
I’ll lay fourteen of my teeth (and yet, to my teen
be it spoken, I have but four) she’s not fourteen.
How long is it now to Lammastide?
I’ll look to like, if looking liking move.
But no more deep will I endart mine eye
Than your consent gives strength to make it fly.
A man, young lady—lady, such a man
As all the world—why, he’s a man of wax.
No less? Nay, bigger. Women grow by men.
Though his face be better than any man’s, yet his leg
excels all men’s, and for a hand and a foot and a
body, though they be not to be talked on, yet they
are past compare.
I must another way,
To fetch a ladder by the which your love
Must climb a bird’s nest soon when it is dark.
I am the drudge and toil in your delight,
But you shall bear the burden soon at night.
If I profane with my unworthiest hand
This holy shrine, the gentle sin is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.
Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims’ hands do touch,
And palm to palm is holy palmers’ kiss.
O, he’s a lovely gentleman!
Romeo’s a dishclout to him. An eagle, madam,
Hath not so green, so quick, so fair an eye
As Paris hath.
Ancient damnation, O most wicked fiend!
Is it more sin to wish me thus forsworn
Or to dispraise my lord with that same tongue
Which she hath praised him with above compare
So many thousand times? Go, counselor.
Thou and my bosom henceforth shall be twain.
Juliet is the epitome of innocence - she is very young, very sheltered, and knows very little of the ways of the world. Nurse, on the other hand, has "been around the block" a few times. She has been married, had a child who died (Susan), and definitely has a worldly air about her. Nurse jokes about very earthy, bawdy subject matter that goes over Juliet's head as Juliet is so young and inexperienced. Nurse also provides comic relief in the midst of so much youthful passion.