Rye Kvass

1 cup boiling water
1 cup rye flour
1 tsp. sugar
1/4 lukewarm water
1 pkg. dry yeast
4 qts. boiled water, lukewarm

Pour boiling water over flour and stir briskly until smooth.
Cool to lukewarm.
Dissolve sugar in the lukewarm water, sprinkle yeast over it
  and let it stand for 10 minutes.
Combine with the cooled batter, cover and let it rise
   until light and double in bulk.
Stir into it 4 qts. of boiled water cooled to lukewarm.
Cover and let stand in a warm place for 3-4 days.
When liquid is clear and tastes mildly sour, pour it off
   carefully without disturbing the bottom layer.
Cover and keep refrigerated.
Use in borch or chilled soups.

This recipe is taken from an Ukranian cookbook from the 50's.  The reason for boiling the water and letting it cool to lukewarm is to kill any wild yeasts that might contaminate the sponge.