Illustration of objects with different topologies, distinguished by the holes of different dimensions present in them. A disk (panel (a)) is characterized as a single connected object, while a ring (panel (b)) is characterized as a single connected object that forms the boundary of a hole. A sphere (panel (c)) is different from both a disk and a ring, as it is a single connected surface bounding the 3D cavity in the interior, while a torus (panel (d)) is characterized by a surface in the shape of a hollow tube that surrounds a visible hole, as well as the hole in the interior of the tube body, which also doubles up as a cavity or void.