Thouless showed how this effect was related to topology. Consider the bread loaf, bagel and pretzel again. The loaf has no hole, the bagel has one and the slightly looped pretzel has two. Their number of holes varies by a whole number. It can’t vary by a fraction. There is no bread with one-and-a-half holes, or two-and-a-third holes. So any change in the number of holes can vary only in integer leaps — just as the conductivity changes in the quantum Hall effect. (In 1988, Haldane showed that a similar effect can occur even in the absence of a magnetic field.)