Water is a natural substance composed of the chemical elements hydrogen and oxygen. It exists in many forms, including gas, liquid and solid states. It is naturally tasteless and odorless, and serves a base for many other substances and compounds. Like air, water is also plentiful on Earth, and is essential to all life forms. For human beings, water helps to protect tissues, ensure the proper functioning of the spinal cord, brain and joints, and aids in the digestive process and cardiovascular functions. Because so much of the body’s internal operations rely on water, lack of water can be fatal in just as few as three days. In fact, 70% of the body’s overall physical mass is made of water. It is recommended human beings drink at least 8 cups of water depending on their body weight and mass. While other compounds and substances include water (such as soda, tea, and other beverages and foods), 3-4 liters of clean, unaltered water is necessary for optimal health. In addition to providing internal nutrition, water also provides the basis for external hygiene. Habits and practices such as bathing, showering and washing hands help to remove harmful bacteria and prevent disease. On average, it is recommended human beings bathe or shower 5-7 times per week, and wash their hands daily after using the restroom, before eating, and whenever entering their homes from outside. Certain occupations may require more frequent handwashing, particularly those that include working with food or in the health care industry.