Kefir is a thick cheesy dairy beverage similar to buttermilk or yogurt, like kombucha made with milk.
This is the easiest way I know to preserve milk without refrigeration.

Mongolian "airag" may be the same thing.
I learned about kefir from Russians. They say "kee-fear", rolling the 'r' just a bit. Both syllables last a bit longer than you'd like and are accented equally.

Kefir will not "breed true" without a "mother". Also called "kefir grains".
That's a distinct colony which looks (and feels) like a little brain floating in the yogurty stuff.
You save and transfer it from batch to batch, like Kombucha.
The mother grows slowly from batch to batch. Kefir with no mother will not generate one.

The mother contains a diverse population of microbes that get along well. They can out-compete the wild organisms and don't need the milk scalded first.
Whereas yogurt needs to be held at certain heat to yogurtize properly, kefir can be brewed at room temperature or even saddlebag temperature.
One of the nice features of Kefir is that if it separates and settles, leaving clear whey, you can stir it up and it won't separate again. Yogurt by contrast will settle again.

My dad got a mother from Russian friends at his Orthodox church.
He's given me mothers a couple of times. Other Russian friends have given me mothers also.
I can't seem to keep them alive.

So there I was stranded with no starter. So I bought some commercial kefir, innoculated a kombucha mother with it, and have been using that to make "kefir" for a year and a half now.
It's a little bit different from purebred kefir, but the mother is much more durable.

If you have a real Kefir mother and want to know how to "operate" it, skip ahead to step 3.
If you only have dried mother grains, find the activation info elsewhere.

Step 1: Get a Chunk of Kombucha Mother

Anyone who brews kombucha will be happy to provide a chunk of "mother".
If you can't find such a person, buy a bottle of commercial kombucha.
Pour it into an open jar and tie a cloth over the top.
If it's still alive you'll see a skin forming on top in a few days.
After a month or two you'll have a nice thick layer of mother in there.

Kombucha gets more and more sour with time. The more sour it gets the better the mother grows.
Here's a nice big chunk from a kit I gave a grad student friend. After graduation the offices were demolished and I found the jar among the rubble with a thriving mother inside.
The other layers in this jar are thinner. Each represents a few weeks' growth.
When I decant and brew again the mother doesn't always float at the surface and a new layer forms atop that.

Step 2: Innoculate Your Mother

Put your mother in a jar and pour in some commercial kefir.
Swirl it around. Leave it for a couple of days.

You now have a hybrid kefir/kombucha mother.

Add milk and follow the next steps to make batches of kefir.
I continued to drench the mother with commercial kefir prior to adding milk until the bottle ran out.

Step 3: Meet the Cast

On the left we have a tub of kefir ready to eat.
There's a cloth over the top instead of a lid.
That's very important for two reasons.
1: Let in air. We don't want anaerobic fermentation.
2: Prevent condensation. Anything looking like a lid will get condensation, even if it's loose. Then there will be white mold. Then your mother will die and fall apart.

On the right we have an empty tub. Offstage we have a lid that fits it.

In the background is a jug of cheap factory famed milk.
Sometimes I buy organic milk right from the farmer's bulk tank right after milking.
That's so much better I want to run around singing and hugging people.

Today I'm far from the farmers I know so I'm hoping kefir critters eat hormones.

Step 4: The Curtain Rises

Reach into the kefir and pull the mother out.

If it were a real kefir mother and you weren't going to make more right now you might put it in a jar of water in the refrigerator to rest.

Step 5: Transfer and Pour

Transfer the mother to the new container and pour milk on it for the new batch.
If this were a real kefir mother you would wash the curds out from between the brain's little lobes.
You might take it out and do the same with water before replacing it in the milk.
Retained curds from the last batch can result in kefir that cracks and settles prematurely.
Also it can retard your mother's growth.

With this massive hybrid mother though, I don't worry about any of that.

Step 6: Exit Stage Left and Right

Cap the old batch and put it in the fridge.
You could leave it out but it would continue to get sour.
Rubberband the cloth cover on the new batch and put it on your "living foods" shelf.
Wash the cloth every few batches or it will get moldy and transfer mold to the tub.

Kefir is best on the second or third day of brewing, just as soon as it sets up.
It's okay to agitate the vessel, especially if any separation occurs.

Real kefir that I've made from brain-shaped mothers has a very buttery taste on the second or third day of brewing, and often has a carbonated sparkly taste.

This kefir never really goes through those phases, or goes through them fast because this mother is so large. A sharp lactic acid sour dominates the flavor, which is what I'm after at the moment for making "Budwig Formula".

This kefir won't really spoil, it will just get more and more sour.