Camels have humps on their back filled with fatty tissue, not water. But one gram of fat is equivalent to one gram of water when metabolized. Storing fat in their humps, instead of having the fat distributed evenly throughout their entire body, allows the camel to keep cool. Otherwise, the overall body fat would act as insulation and cause extreme heat within the camel's body. The fatty tissue in the hump is a source of energy and water that the camel uses when needed. A camel's hump will swell when it is packed full of fat and deplete as the stored fatty tissue is used to hydrate and energize the body.