Ice floats. That is, water expands when it freezes, and thus becomes less dense. For most other stuff, that is not the case. Most of the time, when something goes from a liquid state to a solid, it actually becomes denser. Water is an exception.
So what? We all learned in school that an iceberg has 90% of its mass under water and 10% of its mass above the surface. Big deal, nothing new here.
Well, suppose ice did not float. Then (obviously) it would sink to the bottom. That's not good. If this happens in the ocean, the sun will not be able to melt this ice again. So, over time, there will be more and more ice sinking to the bottom of the ocean. Until the entire ocean is frozen. Oops. That's bad. Very bad. Life could not have formed on Earth. Kind of obvious, when you think about it, that is.
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