Fig. 411.05 Four Spheres Lock as Tetrahedron:
 A single sphere is free to rotate in any direction.
 Two tangent spheres although free to rotate in any
direction must do so cooperatively.
 Three spheres can rotate cooperatively only about
respective axes which are parallel to the edges of
the
equilateral triangle defined by joining the sphere centers,
i.e. each sphere rotates toward the center of the
triangle.
 Four spheres lock together. No rotation is possible,
making the minimum stable closestpackedsphere
system: the tetrahedron.
