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Released from her physical exterior, Penny is revealed to be an ethereal, shape-shifting being made of pure emotion. Overwhelmed by insecurity and her father's disapproval, she begins to transform into various creatures based on her shifting moods—ranging from a giant rampaging beast to a small, timid mouse. Gumball spent the remainder of the episode chasing her through Elmore, ultimately proving his affection is for the person inside, not the shell she wore. Why "The Shell" is a Fan Favorite The Amazing World of Gumball - Season 3Eps20