The polyhedron shown on the right in Fig. 1.1 is not convex and does not lend itself to the above argument. However, if we think of it as being made of rubber then we can easily imagine how to deform it into an ordinary round sphere. During the deformation we stretch and bend the polyhedron at will, but we never identify distinct points and we never tear it. The resulting corre-spondence between the points of the given polyhedron and the points of the