Answer:
Lenny kills Curley's wife by snapping her neck after shaking her very aggressively
Explanation:
Curley's wife goes into the barn to help comfort Lenny after he has killed his puppy. She tries to comfort him by allowing him to play with her hair. She ends up laughing, joking with him to let her hair go until she gets serious, and tries to pull away from him. Lenny, not knowing how strong he is sometimes, pulls her back and shakes her in order to play with her soft hair, her neck snapping in an instant.
