Water. Without it, life could not exist. Over 70 percent of the planet is covered in water. Water comprises a remarkable 60 percent of the mass of most living things. What are the properties of water that make it essential to life? For one thing, water is one of the only substances that can exist in all three of its physical states - solid, liquid, and gas - at temperatures that are found naturally on Earth. But there are many other properties of this miraculous compound that make it particularly well suited to life. In this module, you will explore some of them and learn what it is about the unique structure of water molecules that make water the elixir of life.