Abstract: A tumbling toy includes a shell or housing formed of two or more telescoping sections, providing adjustment of the length of the shell and thus the moment of inertia of the toy to affect the tumbling rate. Extending the telescoping housing increases the moment of inertia of the toy, and provides a longer path of travel for the mass within the shell, resulting in a slower rate of tumble for a given slope. The sections may be separated also, allowing the mass contained within to be exchanged for one of greater or lesser mass, which also affects the tumbling rate of the toy. The outer shell sections may be made of plastic or other suitable material, with the moving mass contained therein comprising a marble or the like, or a steel or other metal ball bearing or the like, or a lead sphere or other suitable shape. An internal mass of less than the mass of the shell structure may be provided if desired, as well as masses substantially equal to the shell, or greater than the shell, as desired.