What did George tell Slim about Lennie's strength?

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.