Abstract: This doll, or mannequin, features a relatively rigid core with an expandable flexible covering and inflation mechanism in order to expand the flexible covering to simulate weight gain or muscular development. The inflation mechanism may include a valve to slowly release accumulated pressure to simulate weight loss. The flexible covering, simulating a person's skin, may be formed integrally upon the core or may be placed upon the core as a close-fitting enclosure. The muscle gain doll features nonexpanding bands attached to the flexible covering to simulate muscular definition. Such a doll may be used with essentially non-stretchable clothing to simulate a person growing larger relative to their clothing and the unseemly appearance resulting therefrom.