sentences:  Another characterization states that if "d1", "d2" and "d3" are the common perpendiculars of "AB" and "CD"; "AC" and "BD"; and "AD" and "BC" respectively in a tetrahedron "ABCD", then the tetrahedron is a disphenoid if and only if "d1", "d2" and "d3" are pairwise perpendicular.The disphenoids are the only polyhedra having infinitely many non-self-intersecting closed geodesics.On a disphenoid, all closed geodesics are non-self-intersecting.