Kefir Grains are not actually grains at all, nor are they, as some will say, a mushroom species found in the mountains near the snow lines.
They are a mixture of bacteria and yeast living in a symbiotic matrix that clump together. Under a microscope they look like small cauliflowers.
They are used as a starter-culture for Kefir.
To rehydrate Kefir Grains, soak a tablespoon of Kefir Grains in two teaspoons of water for 5 to 6 hours (the water just needs to be lukewarm when you start. Wild-eyed granola types insist that the water should be spring water drawn from wells from which only virgins have drunk.) Then rinse in cold water, and place in half a cup of fresh milk (4 oz / 120ml). Change the milk every three hours until the grains have turned from yellow to white, at which point they are ready to be used to start Kefir with.
After use, Kefir Grains can be washed and reused, or dried again for future use.
Kefir Grains can be dehydrated, stored, and then rehydrated when wanted again.
There's no doubt that Kefir Grains are a very old culture that mankind has kept alive and passed along. Some say it originated about 3000 BC, which does rather seem stretching it in terms of something kept going so long without dying out, mutating or evolving.
Others stories repeated breathlessly hold that Kefir Grains were a gift from Allah to Muslims in Central Asia. Given, however, that Muslims are forbidden from drinking alcohol, or even from using normal vanilla extract because of the alcohol content in it, it does seem highly unlikely that Allah would give them something which made a beverage with up to 2% alcohol content in it.
Literature & Lore
There is much mysticism in the West attached to Kefir Grains by Health Foodies.
Celebrate Vanilla Pudding Day by whipping one up from scratch or from a powdered mix, or by just whipping open a pudding pak on the run. Vanilla pudding is not really known in England, the closest thing is custard, but never mind: they have crème caramel, double cream and blanc mange.