Symbolism, Imagery, Allegory
In Stonetown Harbor, there's a bridge that connects the mainland to Nomansan Island, and the kids have to cross it in order to get to the Learning Institute for the Very Enlightened (or LIVE, or—backwards—EVIL). On their way there, "they were apprehensive. The closer they came to the island, the more real the danger felt" (10.3). And indeed it is on the island that the children learn things they "rather missed not knowing" (20.1).
In the end, of course, they return to the mainland and to a simpler—and less evil—way of life. But check this: when they go back to the mainland, where they all started as innocent kids who had no idea there was an evil guy with an evil machine and evil plans, they don't go back across that bridge. They leave the island by boat, and even though the station wagon stops to pick up Mr. B on the mainland side of the bridge, the kids never set foot on it again. Why? Because that bridge has been crossed. Literally and metaphorically.