Abstract: A monolithic ball structure made of resilient material and consisting of two equal diameter spherical sections connected by a neck portion of reduced diameter. The neck portion is defined by a segment of a surface generated by the revolution of a planar arc about an axis lying in its plane. The axis in this particular structure coincides with the common axis connecting the center points of the two aforementioned spheres. The radius of the arc is one-eighth of the diameter of the sphere in one particular embodiment thereof. The resilient material has a Durometer hardness value in the range of 40 shore A to 60 shore A.