Free radicals are highly reactive atoms that have at least one unpaired electron. They bind with the electrons of other chemicals, often setting off a chain reaction of molecules losing and gaining electrons. Their "stealing" of electrons can disrupt important processes and cause damage to cells. Cell damage caused by free radicals is called oxidative stress. Free radicals are normal by-products of the metabolism that are usually mopped up or "quenched" by antioxidants such as vitamin C. The formation of free radicals may also be promoted by radiation and some toxins.