Kvass, a fermented drink made from black bread, is popular in many Eastern European countries, but it is particularly associated with Russia. Kvass is made from rye bread or white bread, water, and yeast, and is sometimes flavored with fruits, herbs, or spices. It is typically low in alcohol (less than 1%) and has a tangy, slightly sour taste. Kvass is often considered a health drink and is believed to aid digestion and boost the immune system.