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Why does a massive ship float but a tiny rock sinks? The answer is density. Density is how much stuff is packed into a space. Scientists define it as mass divided by volume. More mass packed into the same space means higher density. Imagine two boxes the same size. One filled with feathers… the other with rocks. Same size, but the rock box is heavier, so it has higher density. That’s also why oil floats on water and hot air balloons rise. Lower density things float on higher density things.