Glancing through George Gamow’s popular excursion into modern biology Mr. Tompkins Inside Himself, written with Martynas Yeas, my eye fell on an amusing account of parity. In it a matador faces a bull in a Möbius ring. He would find, should he run right round the Möbius ring by-passing the animal, that he would face the bull again, but the wrong way up, doing, as it were, a hand-stand. The matador, the authors point out, changes his parity. And standing on one’s head is no way to fight a bull.