The Bell in the Bridge

When Charlie visits his hardworking grandparents in the summer, he's often left to himself. One day he goes out to play by the stream with a tin can for tadpoles, a special weed-whacking stick, and stones to drop from the iron bridge. He notices that when he strikes the bridge with a big stone, it rings with a bong like a church bell and echoes into the valley. And sometimes a faint, very distant, different-sounding bong comes back.