A steel bar dropped into water sinks, but a boat made of steel floats. Why? Because most of the space in a boat is taken up by air. Taken as a whole, this makes the boat much less dense than the steel bar, and even less dense than water. Objects of lesser density float on liquids of greater density. This is also the reason why holes in the bottom of a boat cause it to sink: as air floods out of the boat, the overall density of the boat increases to the point where it's more dense than the surrounding water, at which point it goes down.
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