The correct answer is a free radical. Free radicals are unstable atoms in the body that can damage cells, causing illness and aging. They occur from a process known as oxidative stress. Oxidative stress occurs when an oxygen molecule splits into single atoms with unpaired electrons. Electrons like to be in pairs, so these atoms (free radicals), scavenge the body to seek out other electrons so they can become a pair. This causes damage to cells, proteins and DNA.