The big thirst

The water coming out of your kitchen tap is four billion years old and might well have been sipped by a Tyrannosaurus rex. Rather than only three states of water -- liquid, ice, and vapor -- there is a fourth, "molecular water," fused into rock 400 miles deep in the Earth, and that's where most of the planet's water is found. Unlike most precious resources, water cannot be used up; it can always be made clean enough again to drink -- indeed, water can be made so clean that it's toxic. Water is the most vital substance in our lives but also more amazing and mysterious than we appreciate. As…