Look in thy glass and tell the face thou viewest
Now is the time that face should form another;
Whose fresh repair if now thou not renewest
Thou dost beguile the world, unbless some mother
For where is she so fair whose uneared womb
Disdains the tillage of thy husbandry?
Or who is he so fond will be the tomb
Of his self-love, to stop posterity?
Thou art thy mother's glass and she in thee
Calls back the lovely April of her prime;
So thou through windows of thine age shalt see
Despite of wrinkles, this thy golden time
The young man is like a mirror to his mother in which she can look at to see a reflection of herself as she was in her youth – and the mother is like a mirror to the child in which he can look at and see a reflection of himself as an adult. 
 This line is remarkable for its neat conciseness, characteristic of Shakespeare’s skill with words. 

Sonnet 3