In "The Moose and the Sparrow" one young man, nicknamed Sparrow, is relentlessly and cruelly bullied by an older man, nicknamed Moose. Both men work in a lumber camp. Moose bullies Sparrow because Sparrow is smart and smaller than him. At first Moose makes fun of the younger man's real name, Cecil, "mouth[ing] it with a simpering mockery, as if it pointed to the kid being something less than a man." The bullying thus starts with verbal mockery, which Sparrow ignores rather easily.
A few weeks later Moose and some of his friends take Sparrow while he is sleeping and throw him into a river. Moose also uses tar to paint a moustache on Sparrow's face. Neither of these incidents endanger Sparrow's life, but they do nonetheless represent an escalation of the bullying. The bullying is now physical, whereas it used to be only verbal. The physical bullying is more humiliating and degrading than the verbal abuse.
Moose's bullying of Sparrow becomes so frequent and so serious that Sparrow comes to think that one day Moose might just kill him. Indeed, he exclaims to another of the characters in the story, "He wants to kill me!" He also believes that if he, Sparrow, decides to leave the lumber camp, Moose will make sure that he leaves "crippled."
The author builds the tension of the story by emphasizing the frequency and escalating extremity of Moose's bullying. The more Moose bullies Sparrow, the more the tension between the two characters builds and the more we feel that eventually Sparrow will, as it were, snap and lash out against Moose. This is of course what he eventually does do. The tension becomes too much for Sparrow, and, fearing for his life, he decides to kill Moose.