One of the first things you hear when learning about the states of matter is about good old H2O: Steam, Water, Ice.  Naturally you are asked “Why does Ice float?” The answer is a simple matter of density, frozen water is less dense because water expands when it freezes.  You can do a bit of an experiment by filling a balloon with water.  Paint the balloon and put it in the freezer.  When it freezes the water will expand, and so will the rubbery balloon, but the dried paint will crack at weak points. This is exactly the same thing...