For about a year now, I've been making my own Kefir.  About once or twice a week, I refresh the milk in my kefir.  Kefir is a form of yogurt, where beneficial bacteria eat away at the milk sugars creating either a slightly tangy (to some people, just sour) creamy product that is often slightly fizzy from natural carbonation.  There's a lot of detail about kefir that I won't get into since I haven't done my research on it, instead, I'm going to go over what I do each week for more kefir. 

Kefir starts with clumps of bacteria and yeast, usually Lactobacillus along with other strains.  These clumps are called grains and look like tiny brains.  If you're lucky, you can locate someone in your area via different sources, to find where you can get some free kefir grains since people who make kefir often keep getting more and more(more on how they reproduce later).  It doesn't take very many grains to create kefir.

I toss these grains into a jar, about a quart or half gallon jar is enough for me, one person, and I add milk until it's just about full with just enough of an airspace.  I then leave that jar out overnight, (that's right, no heating).  The kefir grains go into action, eating the lactose (milk sugars) and reproducing.  I tend to use a 1% milk, however it's possible to use whole or even cream to create the kefir.  A higher fat milk creates a creamier product, but I like the liquidness of the 1%.

Often, people will use a lowfat milk and warm it with milk powder to create a higher protein liquid which they then create either kefir or other yogurts with.  Some people will even use raw, unpasturized milk to create kefir.  I've also seen other forms using goat milk and what seems to be coconut milk.  I'm not quite sure how that works, however there are apparently forms of kefir that work with just juice too. 

I have heard that the forms of kefir that work with juice (known as water kefir) may be a different strain of bacteria that's still beneficial, but instead of lactose, they eat the sugars (fructose, glucose and probably dextrose) leaving a slightly fermented juice that again, can be slightly fizzy. 

The carbonation comes from the creation of carbon dioxide from the bacteria in the kefir which fills the airspace and eventually equalizes into the liquid so that there's equal carbonation in both.  I've done this longer than beer, but I suppose it's the same process.

After a day, maybe even two if I forget, I strain the liquid into another jar, leaving the grains behind.  I stick this jar into the fridge and pour fresh milk into the container with the grains.  If I wanted to, I could again leave the grain  jar out to create more kefir, but because one a week is enough for me, I'll stick both bottles into the fridge.  Once cold, I'll drink my kefir either plain, with honey, or with chocolate syrup.  I'll even use it in place of yogurt or milk (or even in addition to yogurt) when making a smoothie.

Extreme closeup of grains and a few curds

Kefir!