Water is the most familiar substance in our lives, and the most important. Every cell in our bodies is plumped full of water; every heartbeat, every thought � including thoughts about water � happens only because of the cascade of chemistry that water makes possible. Yet we know almost nothing about water, starting with such basics as: Where does our water come from? Where does it go once it disappears down the drain? In fact, even what we think we know about water is often wrong.