"Try it yourself. Go out, pick somebody and watch them. Take your phone and a notebook. Persist. What begins as a confluence of yours and another person's journeys, on the train maybe or leaving a cinema, gets into an entanglement. You follow, feeling that it's not really following because you're going the same way, then when they at last reach their office you feel the clutch of a goodbye. It's normal. But how many times do you think the person being...