When George, Slim and Lennie find themselves in the bunkhouse together in Chapter 3 of Of Mice and Men, George and Slim have been talking about Lennie's childhood and youth because it has been learned that Slim and Lennie are from the same hometown and that Slim knew Lennie's aunt, who entrusted Lennie to George's care at her death. While Slim and George are talking, Lennie enters hiding a puppy that Slim has given him. Seeing this, George orders Lennie to return the puppy to Slim.
Lennie's behavior with the puppy, his clutching manner and his efforts to hide it, when combined with Slim's past knowledge of Lennie and George's explanation of what forced him and Lennie to leave their previous job in Weed, all convince Slim of George's childlike mentality. After watching George with the puppy, Slim recognizes George "ain't mean" and that he's a "nice fella," but one who has a limit on his understanding and intelligence that makes him childlike.