The Mishnah taught that there are six grades of ritual baths, each higher than the other. The first is rainwater in a water hole. Superior to that is the water of rain drippings that have not stopped. Superior to that is the water of a mikveh containing 40 "se'ahs" of water, for in such a mikveh persons may immerse themselves and immerse others. Superior to that is the water of a fountain whose own water is little but has been increased by a greater quantity of drawn water. Superior to that are salty or hot waters from a spring, which can render clean when flowing. And superior to that are living waters, which serve for the immersion of persons who have a running issue and for the sprinkling of persons with skin disease, and are valid for the preparation of the water of purification.