Water is absolutely essential. You could probably survive for weeks without food. But you can live only a few days without water. Humans have no capacity to store “spare” water, so we must quickly replace any that’s lost. Overall, water makes up between 45 and 75 percent of a person’s weight. Leaner people have proportionately more water because muscle tissue is nearly three-fourths water by weight, whereas adipose tissue is only about 10% water. The one bit of chemistry that almost everyone can rattle off is the chemical formula for water: H₂O. Water is such a simple molecule that people often do not appreciate its extraordinary physical and chemical properties.