Complex sentence: Ted Wiggins is an idealistic 12-year-old boy, who lives in "Thneedville," a walled city in which everything is artificial and made of plastic, metal, or synthetics.
Split into simpler sentences: Ted Wiggins is an idealistic 12-year-old boy who lives in "Thneedville". Thneedville is a walled city that, aside from the human citizens, is completely artificial; everything is made of plastic, metal, or synthetics.