"In a Station of the Metro," has to be one of the shortest poems I've ever enjoyed. It's basically a place and two metaphors.
In the Station of the Metro
The apparition of these faces in the crowd;
Petals on a wet, black bough.
Because the poem is only two lines, the title acts as a third line, and also is critical to form the first concrete image that the two lines of the poem use as their base on which to layer meaning and metaphor.
The first line of the poem transforms the image of the metro into one of a sea of ghostly, white or translucent faces, the second line introduces the final conflict, a transformation of the faces into an image of nature that is so divorced from the very modern, industrial underground train station.
In this conflict of image builds the opportunity of great emotional insight: even modern man is natural and beautiful at the height of his technological achievements, in a concrete hermetic bubble that has no room for organic trees and flowers. And yet at the same time, the image is one of transience and fragility, of flower petals that are rain-slicked, and this brings the feeling too at once of freshness and of the petals that are scattered even by a light storm.
Imagist poetry, sometimes pretty bad-ass.
Can you write a three line imagist poem? Take a scene that moved you, describe its literal location. Then the first metaphor, or element of the scene that appeals to you as metaphoric. Finally, think of the opposite emotion, opposite surroundings, opposite scene. Compare them. You have a three line poem full of conflict and most likely, ripe with emotional windfall.
Notes from Pound on the experience of writing the poem.