Why does London wait until after lunch for the man to fall into the spring?"To Build a Fire" by Jack London
London may have waited until after lunch for the man to fall into the spring because after his belly was full, he would perhaps have been more complacent and satisfied, not being as careful as the old-timer had cautioned. We already know that he is not native to this land, but a newcomer, and that he has an intellectual understanding of the dangers that face him, but not an instinctual understanding, like the dog.
The second reason London may have waited until after lunch is that before the man can properly eat, he must build a fire. As London describes the process of the fire building, we can envision what he must do. Therefore, by the time the man falls into the spring and begins to freeze, the suspense while we wait and hope for him to build a fire is more intense. We have already learned, as "newcomers to the region" ourselves, how imperative that fire is. In his predicament, we can imagine the panic of an unexperienced traveler in this unwelcoming, dangerous environment, and appreciate with our own desperation on his behalf, how his thoughts start to become scattered as he fights for his very life. That one element, the building of a fire, becomes the detail that drives the plot along.
After all, the story is entitled, "To Build a Fire," and the focus is about the fire and survival. It is only after the first fire is built that we can appreciate its importance in surviving in the frozen Yukon territory.