And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gaz'd on now,
Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
To say, within thine own deep sunken eyes,
Were an all-eating shame, and thriftless praise,
How much more praise deserv'd thy beauty's use,
If thou couldst answer 'This fair child of mine
Shall sum my count, and make my old excuse,'
Proving his beauty by succession thine!
This were to be new made when thou art old
And see thy blood warm when thou feel'st it cold
So when he makes a kind of copy of himself by bringing a child into this world he will keep on living He will feel warm inside and his beauty will not be futile even if time damages his body. 
 The final line is neatly balanced, with “warm” matching its opposite, the “cold” — an example of  chiasmus .
Sonnet 2