Lennie was in the barn playing with the puppy that he had killed by an accident.
"You wasn't big enough"
Then walks in Curley's Wife in her bright cotton dress, being flirtatious he then tried to hide the dead puppy from her and she tried to talk to Lennie but George told Lennie not to have conversation with Curly's Wife and as Lennie was told he listened.
Then Lennie runs away into a bush and leaves Curley's wife there in the barn
but she insisted and egged on a conversation and started making conversation asking questions getting Lennie to open up to her
Her knowing that Lennie loves softs things she urges him to touch her soft hair and he does and then he GRIPS HER HAIR AND DOESNT LET GO !
He starts to cover her mouth because she starts to yell and he doesnt want to get into any trouble and he shakes her head so hard and so much he snaps her NECK !!!