The average human has 40 billion fat cells. These tiny, glistening, oleaginous buggers are designed to store energy for when we need it. Along with the brain, the liver, the pancreas and the stomach, fat cells manage our energy needs as well, maintaining constant communication through our blood system. This system is highly efficient, and evolved over millions of years, during most of which humans were always looking for food.
And long before the invention of the cheeseburger.