The child-like Lennie loves to pet creatures such as the mouse he previously had in his pocket, in the clearing, when he and George camped out the night before coming to the ranch. Now that they are working at the ranch, Lennie learns that Slim, the muleskinner, has a dog who has had puppies, and, like a young boy, Lennie loves to pet them.
George has to caution Lennie not to pet the puppies too much or interfere with the mother. Slim remarks, "He's jes' like a kid, ain't he." George replies, "Sure he's jes' like a kid. There ain't no more harm in him than a kid neither, except he's so strong." This passage about petting the puppies and his being so strong foreshadows Lennie's actions with Curley's wife who tells him, "'Feel right aroun' there an' see how soft it is.'"
Lennie's big fingers fell to stroking it...."Oh! That's nice..."
"Look out, now, you'll muss it...Let go....You let go!"
When Lennie sees that she does not move, he whispers in fear, "I done a bad thing. I done another bad thing" as he associates what happened to his pup with what he has just done to the wife.
George lets Lennie stay and sleep out in the barn with the puppies; Lennie even wins the mother's trust so that he takes them in and out of their nest repeatedly. When the hands are playing horseshoes, just before Curley's wife comes to the barn, we learn what Lennie has done to his puppy, the white and brown one he wanted. Lennie's comments to himself tell us that he bounced his puppy and that the puppy wasn't big enough and that he has killed his puppy, just like he killed the mouse and just like he would kill Curley's wife. Addressing the dead puppy, Lennie says:
"Why do you got to get killed? You ain't so little as mice. I didn't bounce you hard."