Usually we cook meatballs with aluminum pot, which will turn black. There must be moisture in the process. As long as there is water, aluminum pot will dissolve many substances that we can't see with the naked eye, the most common ones are calcium salt, magnesium salt, followed by iron salt. Different water sources contain different iron salts, which make aluminum pot black. If meatballs are put in aluminum pot and dyed black by aluminum pot, don't eat them any more, because people who eat iron salt directly will get food poisoning, so don't eat them. If there are black spots on the surface of meatballs, it proves that meatballs begin to rot and deteriorate. At the beginning, the surface of the meatball is black and round, and it will spread to the whole meatball after 3 to 5 days, and it will be rotten, black and smelly. You can't eat in this situation.