About thirty years ago, I suffered from severe back pain. For some weeks I lay in a body cast, dazed by pain-killers and muscle-relaxants. When I was recovering, I decided one day that I needed exercise. Very gingerly I got on my bike and, feeling rather sorry for myself, rode slowly up Mundy Pond Road. I drew abreast of a group of boys going home from school for lunch. One of them was holding ...