Imagine that my wife and I walk into our living room one morning to find that our son’s toy box has fallen over, and pieces of BRIO train set track lie jumbled on the floor. But eight of the pieces are joined together in a perfect circle, lying on the floor, with a train on top of the tracks. My wife asks me: ‘Did you make that railway?’ I say ‘No – it must have been you’. She says it wasn’t her. We look at each other for a moment, thinking and then we come to two different conclusions.