So let's say you have a lot of old, stale rye bread. You could make bread crumbs with it, or even a bread pudding. Or you could do as the Russian's do and brew a low alcohol beer with it! Kvas (or Kvass) is a 0.5 - 1.5% ABV beverage that's malty, tart, and refreshing. It's also a staple in rye-loving Eastern Europe to this day, where it's still a cooling summertime classic.
It's a simple beverage to make and there's little variation in the different recipes I looked at. Toast a vague quantity of bread, such as "half a loaf" or "8 slices," and pour on boiling water. Allow this to cool to body temp and then strain out the water from the soggy bread. Stir in some sugar or molasses, then sprinkle on bread yeast. Cover and allow to sit for 1-2 days in a warm place, then bottle. Wait for a week, then chill and enjoy. Or try the Baltic method of sweetening the kvas with strawberries or mint.
What's interesting about kvas is that it's tart, biscuity flavor works especially well in the kitchen. It's used as a broth for a number of soups, including the classic salmon and beet green soup Okroshka, which is actually what pushed me to brew my own kvas. That and having several stale pieces of rye bread lying around from my baking experiments.