Water makes up about 60% of an adult human's total body weight.

The amount of water in a person changes based on age and body type. Newborn babies are about 75% water, while older adults may be closer to 50%. Muscle tissue holds much more water than fat tissue. This means people with more muscle usually have a higher percentage of water in their bodies.Different organs need different amounts of water to function. The brain and heart are about 73% water, and the lungs are about 83% water. Even hard bones are made of about 31% water. The kidneys work with hormones to keep these levels balanced every day.An average adult loses 2 to 3 liters of water daily through breathing, sweating, and using the bathroom. If you lose just 1% of your body's water, your brain triggers a thirst signal. Losing 10% of your body's water can cause serious health problems like kidney failure. A loss of 15% to 20% is usually fatal.Water is also necessary for creating energy. It helps break down a molecule called ATP, which cells use for fuel. Water also has a high heat capacity. This allows the body to absorb a lot of heat without the internal temperature rising too fast.