Unless you enjoy a good topological paradox, this may not be for you. It involves turning a torus shaped surface inside out. You have to puzzle out how two rings drawn on the surface appear to link and unlink as the torus is turned inside out – a topological impossibility. This is hard to visualise so, in the 1970s, Anne Goulden made a model out of fabric (illustrated) which helps explain why, in practice, there is no paradox. The paradox has been explained in print, for example in Martin Gardner’s book Mathematical Puzzles and Diversions.