Why is Water Exceptional?

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Water is the only molecule in the natural world which expands when it freezes. And that is not its only unusual feature. It is the cornerstone of all of life on this planet, and maybe others. Water is part of the myths and rituals of civilisations all over the world. But if H20, the one chemical formula just about everyone can recognise, was just a little bit different, life as we know it would not exist. Mike Williams explains why water is exceptional and what that means for all of us. (Photo: Raindrops on a window. Credit to James Beard)