A metaphor is a comparison that does notuse the words "like" or "as." For example, if I say that a little girl "is a doll," I don't mean that she is actually a doll; rather, I am comparingher beauty and cuteness to the beauty and cuteness of a doll.
In "Mending Wall," Robert Frost uses several metaphors:
a. "To each the boulders that have fallen to each.
And some are loaves and some so nearly balls"
The poet is describing stones that have fallen from a stone wall. The poet describes the shapes of the stones by saying that some are shaped like loaves of bread and some are almost the shape of a ball.
b. "Oh, just another kind of out-door game,
One on a side"
The poet says that the activity of collecting the fallen stones is like a "kind of outdoor game."
The central metaphor in the poem is expressed by the narrator's neighbor, who says, "Good fences make good neighbors." The neighbor seems to be saying that fences are like a line that maintains good relationships between neighbors by showing each neighbor where he belongs. The narrator questions whether this is true:
Why do they make good neighbors?...
Before I built a wall I'd ask to know
What I was walling in or walling out
And to whom I was like to give offence.