Science teacher asked this question today and said how do submarines float and sink at the same time.
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Think your question is about density. A less dense thing will float or rise in denser medium. Density is mass divided by volume. A piece of steel will sink in water unless it is used to form a shape and size whose volume displaces more than it's mass. A submarine is a specifically designed craft which can alter it's relative density by either admitting water to it's ballast tanks or expelling it using compressed air.