Kern et al.5 have constructed a 3D material consisting of microscale rings made of a polymer coated with zinc oxide. The zinc oxide is n-doped, which means that electrical transport occurs through negatively charged carriers. a, Shown here is a simplified illustration of the material's structure, in which there is a current (red arrows) passing through the grey rings and a magnetic field (B) perpendicular to the plane of the page. When the rings are not interlocked, a voltage, called a Hall voltage (V), is observed passing from the negatively charged blue rings to the positively charged red rings, in the expected direction. But when the rings are interlocked, a Hall voltage is observed in the opposite direction. b, Kern and colleagues' observations can be explained by considering a simple system consisting of an array of conducting bars connected by electrical wires (black lines) and placed in a magnetic field. When the bars are connected by straight wires (analogous to non-interlocked rings), a conventional Hall voltage is generated. But when they are connected by looped wires (analogous to interlocked rings), the voltage is inverted.