Water is arguably the single most important substance in our lives. It has been called the “universal solvent” and is the fundamental requirement of all life. Virtually all of the water in existence today has been on Earth for billions of years; any “new” water comes only from comets. All the rest is continually being recycled. More than 80% of the Earth’s surface is covered by water, but less than 1% of it is drinkable, non-saline water. Water comprises about 2/3 of our bodies. Most of us could go without food for weeks, but we would die of thirst after only a couple of days. People in the developed nations each use some 123 gallons (466 L) of water every day, and we each drink an average of about a liter a day. The U.S. EPA bases regulatory matters on the consumption-of two liters per day per person.