When we were very young, my father taught us that you could skip any rock you could pick up, if it had one flat surface. He backed up his belief by hefting a chunk of concrete and hurling it sidearm at the river. He got two skips out of the chunk before it plowed into the water, dragging a column of air down with it. The water rushed back in with an echo-y kerplunk like the sinking of the battleship Bismarck. My father was very strong, and he could make a rock fly, skipping it dozens of times. If anyone could sink the Bismarck with a chunk of concrete, it was Dad.