The word topology refers to the contours of a surface or the shape of an object. In mathematics, topology classifies objects by the number of holes they have. A ball is a sphere with no hole, whereas a doughnut, with its one hole, is topologically different. The ball is topologically equivalent to an apple, and a doughnut to a cup, but not to a ball or a pretzel, since going from one topology to another would require a dramatic change, like ripping a hole. For this reason, the topological states discovered in some materials are robust and resist disruptions, unless they are as dramatic as the one mentioned previously.