The reason that a magnet sticks to the fridge is that electronic spins or magnetic moments in the magnetic material spontaneously align or order in one direction, which enables it to exert an attractive force to the steel door. Magnets are one type of material with such built-in order. A topological defect in such a material occurs as a discontinuity in this order, i.e., a boundary region where the order does not seamlessly transition from one area to another. These topological structures form naturally or can be highly engineered in advanced functional materials.