When you are first born, your body does not have much white fat to help insulate and retain body heat; although there are white fat cells, there is not much fat stored in them. Brown fat cells are somewhat smaller than white, are composed of several smaller fat droplets and are loaded with mitochondria, which can generate heat. A newborn baby produces heat (a process called thermogenesis) primarily by breaking down fat molecules into fatty acids in brown fat cells. Instead of those fatty acids leaving the brown fat cell, as happens in white fat cells, they get further broken down in the mitochondria and their energy is released directly as heat. This same process occurs in hibernating animals, which have more brown fat than humans. Once the newborn baby starts eating more, developing layers of white fat, the brown fat goes away. Adult humans have little or no brown fat.