In 1978 my family was living in Boulder, Colorado. One day on our way back from lunch at a pizza emporium, our brand-new AMC Matador dropped its transmission-literally. The damn thing fell out on Pearl Street. True embarrassment is standing in the middle of a busy downtown street, grinning idiotically while people examine your marooned car and the large greasy black thing lying under it. Two days later the dealership called at about five in the afternoon. Everything was jake--I could pick up the car any time. The dealership was three miles away. I thought about calling a cab but decided that the walk would be good for me. The AMC dealership was in an industrial park set off by itself on a patch of otherwise deserted land a mile from the strip of fast-food joints and gas stations that mark the eastern edge of Boulder. A narrow unlit road led to this outpost. By the time I got to the road it was twilight--in the mountains the end of day comes in a hurry--and I was aware of how alone I was. About a quarter of a mile along this road was a wooden bridge, humped and oddly quaint, spanning a stream. I walked across it. I was wearing cowboy boots with rundown heels, and I was very aware of the sound they made on the boards; they sounded like a hollow clock. I thought of the fairy tale called "The Three Billy-Goats Gruff" and wondered what I would do if a troll called out from beneath me, "Who is trip-trapping upon my bridge?" All of a sudden I wanted to write a novel about a real troll under a real bridge. I stopped, thinking of a line by Marianne Moore, something about "real toads in imaginary gardens," only it came out "real trolls in imaginary gardens." A good idea is like a yo-yo--it may go to the end of its string, but it doesn't die there; it only sleeps. Eventually it rolls back up into your palm. I forgot about the bridge and the troll in the business of picking up my car and signing the papers, but it came back to me off and on over the next two years. I decided that the bridge could be some sort of symbol--a point of passing. I started thinking of Bangor, where I had lived, with its strange canal bisecting the city, and decided that the bridge could be the city, if there was something under it. What's under a city? Tunnels. Sewers. Ah! What a good place for a troll! Trolls should live in sewers! A year passed. The yo-yo stayed down at the end of its string, sleeping, and then it came back up. I started to remember Stratford, Connecticut, where I had lived for a time as a kid. In Stratford there was a library where the adult section and the children's section was connected by a short corridor. I decided that the corridor was also a bridge, one across which every goat of a child must risk trip-trapping to become an adult. About six months later I thought of how such a story might be cast; how it might be possible to create a ricochet effect, interweaving the stories of children and the adults they become. Sometime in the summer of 1981 I realized that I had to write about the troll under the bridge or leave him--IT--forever.