A Weeble is shaped like an egg—in order for the physics principles to work as intended, the shape must have a bottom which is a more or less smooth (unfaceted) hemisphere (to allow the Weeble to roll) and from the central vertical axis the shape must be nearly cylindrically symmetrical (that is, any plane cut through the vertical axis line must produce close to the same profile). Next, the shape must be filled with two basic types of unmixed solids, and the volume of the lighter solid must be greater than that of the heavier solid. Next, the overall shape must have constant positive curvature. Next, the relationship between the heavy solid and the light solid must be such that any orientation of the object off of the vertical axis line must cause the object’s centroid to raise and to become offset. Lastly, the object must have only one position in which it can achieve stable mechanical equilibrium.

Combining these characteristics produces a basic Weeble. In theory, it is not possible to have a Weeble with a centroid that is too low to achieve a stable mechanical.

Illustration of the physics principles by which a “Weeble” roly-poly toy operates. A=position of mechanical equilibrium; B=position of mechanical instability; F=gravitational force; y=vertical axis; m1=low-density mass; m2=high-density mass; C=centroid. Note that between positions A. and B. that C raises slightly and becomes off-center.