Drawing of a more elaborate mikveh. The bather at right immerses himself in the mikveh’s main pool; at left is a smaller storage pool, called an otzar, containing undrawn rainwater. A pipe (broken lines in drawing) connects the two pools. When the pipe is opened, even momentarily, the brief contact of the water in the otzar with that in the main pool is sufficient to cause the main pool to become ritually pure.