Water is truly a wondrous, amazing substance - a priceless gift of profound significance to all of life, to which we are intimately and intricately bound. Earth is the ''Water Planet,'' a shimmering blue jewel spinning in the blackness of space, and it is blue because of the precious film of water - mostly sea water - that covers about 71 percent of the Earth's surface. But every living thing also carries a miniature ocean within each cell, so critical it is for the very biochemistry that fuels our physical being. Humans are about 65 percent water (or put another way, there are about 43 quarts of water in a 150-pound person), our blood is 83 percent water, our brain is 75 percent water, and even our bones are 25 percent water. The water in each of us is ancient, cycled for millennia through the sun-driven hydrologic cycle of evaporation, precipitation, infiltration and purification, and ultimately enters us as we drink, eat and breathe. The water molecules now in your bloodstream may well have quenched the thirst of dinosaurs millions of years ago or have even been part of a 3-billion-year-old sea that sustained Earth's early life.