Kvas is an old Slavic drink, made from fermented bread. The first mention about Kvas was made in the 10th century, but nobody knows who and when it was invented, some historian say’s that the drink was made by mistake. But it’s an interesting question, as it was invented before the beer.
At our day Russians adore Kvas. The kvas is the base of cold soups (like Okroshka) and is a great drink when the weather is hot. You can find Kvas in super markets in two versions, the contemporary one (квасной напиток) or the true natural kvas (квас).
Kvas is famous not only in Russia, find an interesting article about The Kvas in US
Did you taste it already?