In topology, genus, roughly speaking, is the number of holes in a surface. Spheres, bowling balls and wine glasses have a genus of 0 and can be represented by quadratic equations. Bagels, inner tubes, and teacups have a genus of 1 and can be described by cubic equations. Humans are more difficult to specify. However, you will certainly increase your genus by one if you have your ear pierced! Different definitions of genus apply to other types of mathematical object such as a curve, a knot, or a set.