Kvass is a alcoholic drink made in Russia. It ends up as a foggy liquid with yeast suspended in it, and has a very low alcohol level.
In households, the most common way to make Kvass was to toast rye bread or dry it out in the oven, and soak it in water allowing it to ferment. These pieces of dried bread are called "sukhari." To sweeten it, honey or fruit could be used, with both being replaced by sugar and lemons when these became affordable to ordinary people. Making it from bread is a bit of a cheater's way to start it, but it was very common: an old Russian household book, The Domostroi, mentions in Chapter 65 making Kvass from "an ordinary soft loaf."
Purists say that it should be started with a piece of dough instead. Some recipes have you make a dough from yeast, malted grain, rye flour, and sugar, which you then set in a slightly warm oven overnight, and you start your kvass from that dough.
There are many variations. Some recipes start with rye grains, a few start with rye bread. Some modern recipes include Kvasses made from fruit with no grain or bread used. Some people believe that you should put 3 raisins in your batch of Kvass, and that when the raisins float, the Kvass is ready to drink.
Kvass is drunk while it is still fermenting. This gives it a bit of a naturally carbonated feel on your tongue. The homemade Kvass is refreshing, with a touch of tartness. During the Soviet era, Kvass would be sold from trucks that drove up and down the streets. This commercial Kvass was sweeter and flatter. Experienced hands always brought their own mug to the truck: if you didn't, you would be served out of one of the few mugs that the salesperson kept on the truck, and rinsed out between customers in a questionable pail of water.
In Russia today, you can buy a concentrated syrup to make Kvass from, called "Kontsyentrat kvasnogo soosla." You add warm water, sugar and some yeast and let it sit for about a day, then refrigerate. It is mostly made commercially now, by large manufacturers such as "Bavariya" and "Polyustrovo." You buy it in 1/3 litre bottles.
Nowadays, (2004) as Russians acquire a taste for Western-style soft drinks, Kvass sales are plummeting.
The Domostroi (a Russian household how-to book from the 16th century) mentions that a housewife should know how to make Kvass. The book was written in the 16th century; the best known version was edited by a priest named Sylvester, chaplain to Ivan the Terrible (Protohierarch Sylvester); his edition dates from 1549.
The knowledge of distilling alcohol seems to have arrived in Russia sometime in the 1300s -- via Genoese from Italy who were living in the Crimea at the time.
Kvass was and is cheaper and easier to make than beer. The alcohol content was so low, that it not only could be given to children, but that it would come in below the taxation radar which caught beer and vodka.