I like to make Kefir out of goat's milk, it's much better than cows milk made into kefir.

But you can make kefir out of almost anything. I bought some "water" kefir grains and use it in coconut water (which I can buy in little juice boxes here). Water-kefir grains are a bit different than ones done in milk, although you can still make milk kefir out of them too.

Anyway, you have to add a little sugar to the water for them to ferment, but it'll get sour from the fermentation and that is when you know the sugar has been digested by the bacteria and it is ready to drink.

Speaking of which, I should go put my grains in sugar water. They need refreshing every now and then even if you're not using them.