"I am the most debated rule in sport. I require a millimetre's precision. Millions argue about me every weekend. What am I?"

Answer: The Offside Rule. The offside rule in football prevents attacking players from gaining an advantage by being closer to the opponent's goal than both the ball and the second-to-last defender. Its fine margins — sometimes decided by a shoulder or armpit — make it one of the most contentious calls in sport.