Slim is the only person on the ranch in whom George confides. He tells Slim that Lennie is incredibly strong but also unusually slow-witted. George explains that he knew Lennie's aunt, and that when she died, Lennie simply began following George around. To illustrate Lennie's strength, George tell Slim the story of what happened in the town of Weed that caused the pair to have to leave. Lennie saw a girl in the town wearing a dress. Since the reader already knows that Lennie loves soft things--to the point that he tries to keep a dead mouse in his pocket so he can "pet" it--it should come as no surprise that Lennie wants to touch the dress. When he does, the shocked girl begins to scream, which causes Lennie to panic and only hold on tighter. George says he had to hit Lennie in the head with a two-by-four to get himm to let go. The girl claimed Lennie tried to rape her, so a posse was quickly assembled to go after him.
This illustration serves the dual purpose of showing George's need for adult companionship outside of his relationship with Lennie of foreshadowing the events that come later in the novel involving first Curley, and then his wife. While Lennie's super-human strength is an asset for securing work, it is the pair's biggest detriment in keeping it.
During his first conversation with Slim in chapter 2, George gets to discuss his relationship with Lennie. Slim comments about how incompetent the other ranch hands are and asks George whether they have ever bucked barley. George responds and specifically mentions Lennie's strength:
"I ain't nothing to scream about, but that big bastard there can put up more grain alone than most pairs can."
George clearly admires Lennie's power, and the compliment pleases both Lennie and Slim. Later, in chapter 3, Slim affirms George's assessment by mentioning that he had "never seen such a strong guy." During this more intimate discussion, George again comments about how powerful Lennie is by stating that Lennie could beat him to a pulp if he chose to, but that he never did. He also mentions the incident in Weed where Lennie was so scared that he held onto a girl's dress with such power that he had to hit him over the head with a fence picket to let go. They had to leave the ranch in a hurry because the girl accused Lennie of molesting her.
When Lennie enters the bunkhouse trying to hide one of Slim's puppies, George tells him to return the dog to its mother because he (Lennie) might kill it. George, in this instance, also gives Slim insight into how Lennie cannot control his strength. Lennie has killed numerous soft, furry animals by fondling them too strongly, and George is afraid that the same might happen to the puppy.
It is ironic that Lennie's greatest asset is also his biggest liability. Lennie does not know his power and is unable to control it. It is this inability that regularly gets him and George into trouble, as with the incident in Weed. George has learned to live with Lennie's inadequacies, but he is, nonetheless, exasperated. Unfortunately, it is this particular shortcoming that culminates in a double tragedy. While fondling Curley's wife's hair, Lennie panics when she starts screaming and breaks her neck. George, in an act of kindness, later kills him so that he may escape Curley's brutal retribution.