Dissolve 1/4 cup sugar in a cup of water in a quart jar. Add 1 cup stale sourdough bread, cubed. Fill the jar with water, leaving 1 inch of headspace. Add a bit of sourdough starter if desired. Cover the jar with air lock lid. (If you don’t have one of these, you need to get one. They are wonderful for fermenting.) Leave to ferment for 4 to 8 days.
Once tangy, strain the kvass from the bread. Refrigerate and/or serve over ice. Lasts 1 to 2 weeks in the refrigerator.