why do we need water?

Humans need to drink water to survive. Your body is approximately 60 percent water, your brain is 70 percent water, and your lungs are nearly 90 percent water. Each day, your body must replace 2.4 liters -- or about 2.5 quarts of water -- through ingested liquid and foods.
Your body uses water in many ways. Water cushions and lubricates joints; nourishes and protects the brain, spinal cord and other tissues; keeps the body's temperature normal; and helps remove waste through perspiration, bowel movements and urination. Humans are composed mostly of water, which is not surprising when you consider that humans descended from single-cell organisms that originated in the oceans millions of years ago.