Two wheels. A frame. Two pedals. What could be simpler than a bicycle? And yet the bike -- old, and cheap, and slightly comic -- continues to inspire a passionate following. Since the millennium its use in Britain has doubled, and then doubled again. Thousands now cycle to work, and more take it up every day. In trial after trial, it is the bike which reaches its urban destination faster than the car, the bus, the underground or the pedestrian. Self-reliant and straightforward, cycling has recycled itself. It is an antiquated idea, and its time has finally come. But what is it about the bicycle that so enchants us? And why do its devotees become so obsessed with it?