Kefir, keefir, or kephir (/kəˈfir/ kə-FEER), alternatively milk kefir, or búlgaros, is a fermented milk drink made with kefir "grains" (a yeast/bacterial fermentation starter) and has its origins in the north Caucasus Mountains. It is prepared by inoculating cow, goat, or sheep milk with kefir grains. Traditional kefir was made in skin bags that were hung near a doorway; the bag would be knocked by anyone passing through the doorway to help keep the milk and kefir grains well mixed.