In the second-to-last chapter of John Steinbeck’s Of Mice and Men, Lennie is in the barn alone playing with his puppy while George and the other men are outside enjoying their Sunday off and playing horseshoes. When Lennie accidentally kills the puppy, this scene foreshadows what will happen in the next and final chapter of the book.
Lennie’s inadvertently violent act casts a pall of inevitability over the story—despite George’s attempts to control him, and despite his best intentions, Lennie is just too big, strong and ignorant to be trusted to act on his own. The novel began with George and Lennie on the run because of Lennie’s mistake, and now it is about to happen again.
Curley’s wife, like the puppy, will also die at Lennie’s hands. The author, John Steinbeck, places Lennie alone in the barn because it is a necessary part of the plot—Lennie needs to be unsupervised to commit his act.
in short terms, he go's into the barn to play with the puppies and then Curley’s wife comes in and talks to lennie and then she asks him to touch her soft hair and he does and he doesn't let go and breaks her neck.