The most abundant substance in your body, by far, is water. About 60% of an average adult’s body weight is water - for a 150-pound person, that’s roughly 10 gallons of water walking around inside you.
Different parts of you contain different amounts. Your brain and heart are about 73% water. Your lungs are around 83% water. Your muscles are 76%. Even your bones, which seem solid, are about 31% water. About the only thing in your body with very little water is your hair and nails.
Babies are even wetter. A newborn human is about 78% water. As you age, you slowly dry out - by your 50s, the average is closer to 50-55%. Water is involved in almost every process your body does: digestion uses it to break down food, blood carries it everywhere, your brain depends on it to function, sweat releases it to cool you down. This is why drinking enough water actually matters: every system in your body needs the supply topped up.
