It is important, she reminded me, to begin with fully-thawed meat and soup bones. Then, the soup pot is filled with COLD (not warm) water, and the apple cider vinegar is added to that. The onion, carrot, and chicken/turkey feet are also added at this time. It should be covered and sit that way for at least a half hour to an hour, so the vinegar can draw the minerals out of the bones. Then, and only then, should the water be brought to a boil.