A metaphor is a comparison that does not use the words "like" or "as." Here are some metaphors from The Lovely Bones.
1) I knew he was going to kill me. I did not realize then that I was an animal already dying."
Susie was not, of course, literally a dog or cat or any other animal. Rather, she is comparing herself to a dying animal.
2) In the walls of my sex there was horror and blood, in the walls of hers there were windows.
A sexual experience does not actually have walls. Susie is comparing her experience to walls that limit and contain a person.
3) Mr. Harvey left his house for the final time while my mother was granted her most temporal wish. To find a doorway out of her ruined heart, in merciful adultery.
A heart does not have a doorway. The author is comparing an emotional release to an exit through a doorway.