The story of the most abundant substance on Earth, from its origins in the birth of stars billions of years ago to its importance in the living world. Water is so ubiquitous in our lives that it is easy to take for granted. The average American uses ninety gallons of water a day; nearly every liquid we encounter is mostly water--milk, for example, is 87 percent water. Clouds and ice--water in other forms--affect our climate. Water is the most abundant substance on Earth, and the third-most abundant molecule in the universe. In this lavishly illustrated volume, science writer Jack Challoner tells the story of water, from its origins in the birth of stars to its importance in the living world. Water is perhaps the most studied compound in the universe--although mysteries about it remain--and Challoner describes how thinkers from ancient times have approached the subject. He offers a detailed and fascinating look at the structure and behavior of water molecules, explores the physics of water--explaining, among other things, why ice is slippery--and examines the chemistry of water. He investigates photosynthesis and water's role in evolutionary history, and discusses water and weather, reviewing topics that range from snowflake science to climate change. Finally, he considers the possibility of water beyond our own hydrosphere--on other planets, on the Moon, in interstellar space.