I was introduced to the idea of probiotic and prebiotic foods when I started reading into more health oriented things a few years back. The topic appealed to me because I was already aware of the idea of "good germs". I took to yogurt well enough, and started researching into what other cultures had, and where yogurt had it's origins, since lots of places seemed to consider it pretty ancestral.
I came across kefir, which sounded pretty appealing because it was similar to yogurt, a more powerfully probiotic beverage and was easier to make.
If this is your first time hearing of the stuff, it's basically a yogurt-like beverage, which is made using a blobby colony of bacteria and yeasts that look like this:
It's sour, slightly carbonated and is great in smoothies or on it's own if you like that kind of flavor.
I did yogurt making for a while but with my set up I'm pretty limited, and it takes a long time. I'd had off-the-shelf Kefir and it wasn't bad, so I thought I'd give it a go. It's real easy - pour milk on the grains and let it sit a day or so. The mixture will separate and that's how you know it's done. Mix it back up, strain out the grains and you are good to go.
Mine tend to go for 48 hours, largely out of habit. This makes it quite sour, and is actually starting to stress my grains because they've reproduced so much over the last couple months. Going to have to start gifting some out! After straining, I typically let it sit in another container for another two days for a smaller, secondary ferment. My other half is lactose intolerant, and this two phase process basically eliminates all of the lactose; certainly enough for her to hand it.
The grains will grow, and speed up the process if you don't increase the milk to match. It's lactic acid fermentation so you can use it or it's whey to make lacto-fermented other things. For example, I've successfully made sourdough bread by using kefir and flour. No starter required!
Good stuff, I recommend anyone give it a try.