The title of the book is a metaphor: Eddie imagines that a giant onion is buried beneath his neighborhood. This onion, he believes, brings sadness and pain to the people, causing them to cry. The vapors that rise off the hot asphalt are evidence of that onion's existence to him.
The sun was climbing over the trees of City College and soon the black asphalt would shimmer with vapors. I had a theory about those vapors, which were not released by the sun’s heat but by a huge onion buried under the city. This onion made us cry. Tears leapt from our eyelashes and stained our faces. Babies in strollers pinched up their faces and wailed for no reason. Perhaps as practice for the coming years. I thought about the giant onion, that remarkable bulb of sadness.
Eddie creates another metaphor when he is talking to his aunt (his tia) later in the story. He says that the "east was one large bruise that was slowly becoming the night." Can you imagine the color of that sunset?
A third metaphor appears in Eddie's statement: "If hard work is the road to salvation, heaven must be packed with a lot of people from Fresno." In this metaphor, he is comparing work to a road that people travel.