Kefir or kephir, alternatively milk kefir or búlgaros, is a fermented milk drink that originated in the Caucasus Mountains made with kefir "grains", a yeast/bacterial fermentation starter. It is prepared by inoculating cow, goat, or sheep milk with kefir grains. Traditional kefir was made in goatskin bags that were hung near a doorway; the bag would be knocked by anyone passing through to help keep the milk and kefir grains well mixed.