Free radicals are atoms with unpaired electrons. According to the free radical theory, radicals damage cells in an organism, causing aging. Mitochondria, regions of the cell that manufacture chemical energy, produce free radicals and are the primary sites for free radical damage. By eliminating free radicals from cells through genetic means and dietary restriction, laboratories have extended the maximum age of laboratory animals. The administration of antioxidants, which eliminate radicals, to laboratory animals fails to increase maximum lifespan.