This paper proposes a simple and efficient method for creating three-dimensional prototypes based on sketching silhouette curves and super-quadric-surface metaballs. Given a silhouette curve, it is first approximated with a set of circles or ellipses. By introducing the third dimensional parameters, a super-ellipsoidal metaball is received for each circle or ellipse. Finally, an analytic implicit surface is constructed by summing the fields of all metaballs and optimizing all parameters. The resulting shape can be conveniently modified by adjusting the positions and shape parameters of metaballs. Different components of a model can be similarly designed by sketching on the different projection planes. This method supports simple solid modeling operations, such as adding, subtracting, semi-sweeping, and is able to generate variants of shapes easily. This method can be applied to designing prototype in the conceptual design stage for computer graphics or computer aided design applications.