What are some examples or quotes from the novel that show Lennie's mental disability?

You should notice Lennie's need to touch soft things. This would be a sensory disability. He continues to rub and stroke the dead mouse even though it is dead. He also has to touch the girl's velvet dress, which gets them in trouble in the first place. Then there is his need to touch the soft hair of the girl on the ranch. He also has a processing disorder. He doesn't understand that the mouse should not be touched because it is dead. He over-reacts when he discovers he is doing something wrong. He grips the little girl's dress instead of letting go. He breaks the neck of the ranch woman when she starts to scream. These show behaviors that are abnormal, more common for a very young child then a grown man.