Kefir is a healthful fermented milk product, a cousin to yogurt. And keeping the cultures to make your own is a little like having a pet.
Kefir (Keh-FEER) grains suggest tiny, rubbery cauliflowers all massed together, but the bacteria and yeast are quite alive. Plop them in a jar and add whole milk (they thrive on fat); amounts up to a quart seems fine. Cover with a cloth and let sit, with a daily slosh to mix things up.
The milk will quickly ferment, and you should have a batch in a day or two. (My batches ferment slower in the winter and faster in the summer, but this won't happen if your house is climate-controlled.) Strain out the grains and refrigerate them; if they're chilled for more than a couple of days, they go dormant and might produce a "bad" batch before the next good one. The culture will also grow over time.
The finished product is a smoothie-like liquid, ranging in taste from sour to tangy -- again, depending on temperature and fermenting time. Kefir is high in beneficial bacteria -- adherents tout it over yogurt for digestive health -- and in calcium, magnesium and B vitamins. Mix with fresh or frozen fruit for a delicious drink; you can also use kefir for yogurt-based dressings, or as a buttermilk substitute in baking.
You can acquire the grains online by searching for "kefir grains for sale." Or simply get some from a friend whose "colony" of little buggers has grown enough to split up.