My wife has started making kefir. It's a yogurt-type liquid that is made by soaking "kefir grains" in milk. The kefir grains are a symbiotic culture of bacteria and yeasts that form these large stringy curds.
Kind of interesting flavor to these, very tasty mixed in a fruit smoothy. You can't make kefir grains from scratch. You have to obtain them. And then you can maintain them in the fridge. And culturing them. My wife cultures them for 24 hours.
Here's a pic of what the grains look like:
