Harper Lee's novella To Kill a Mockingbird contains numerous rhetorical devices. In chapter one of the novella, Scout uses a metaphor to illustrate how they (Dill, Scout, and Jem) could draw Boo out of his house. Jem states that they could draw him out in the same way a turtle is drawn out of its shell.
“Dill, you have to think about these things,” Jem said. “Lemme think a minute… it’s sort of like making a turtle come out…”
“How’s that?” asked Dill.
“Strike a match under him.”
In this example, Jem is comparing Boo to a turtle in its shell. One must only strike a match under the turtle to get it to come out. In the same way, the children must do something to make Boo want to leave his house.
While Harper Lee does not provide a direct metaphor, per say, she does illustrate the similarities between Boo and a turtle--figuratively, both are far too shy to be coerced out of their shells (under most circumstances).