Slim's already been told by George that Lennie's not very bright. But in Chapter 3, Slim realizes that Lennie has the mental age of a child. Slim's dog has just given birth to some puppies, and George asks Slim if Lennie can have one. Slim duly obliges, and George makes it clear just how much this means to Lennie. Lennie loves the puppies so much, says George, that he'd actually go sleep in the barn with them. Having seen how Lennie acts around his puppy, and how he conducts himself on the ranch, Slim realizes that he doesn't mean any harm to anyone. Lennie's actions may often lead to unpleasant consequences, but it's clear to Slim that the gentle giant doesn't have a mean bone in his body.