Traditionally, Kefir is a fermented milk drink that is pleasantly tart and slightly effervescent. Here's a simple recipe for making kefir at home. This recipe is very similar to Doogh (pronounced "Doh") from Persia and Ayran from the Middle East.
-1/3 cup plain yogurt (Greek-style, full fat or low-fat preferred)
-1/3 cup sparkling water
-sprig of fresh mint (optional)
Crush the sprig of mint and place in a glass. Add the yogurt to the glass, then slowly stir in the sparkling water (if you want a thinner yogurt drink just add more sparkling water). Garnish with a fresh mint leaf.
For a more kid-friendly version, omit the mint and start with a fruit-flavored yogurt like strawberry or peach.