Push a water-filled bottle into a jar of water and the level rises — the bottle pushes the water around it out of the way. That’s displacement. The same principle explains why a steel ship floats while a steel nail sinks. Shape aluminum foil into a flat boat and set it on water. It floats. Hammer that same foil into a tight ball and drop it in. It sinks. The ball weighs exactly the same, but it pushes aside far less water, so the water can’t push back hard enough to support it.