I bought some kefir, a fermented milk, from a health food store and since then have been making my own. I don’t strain and rinse the kefir grains; I just spoon some lumps from the fermented milk to a new jar and fill it up with milk. It ferments while protected from dust but exposed to air, as I lay a paper towel over the jar. The resulting mixture tastes something like buttermilk or watered down yogourt. I let it ferment for about two days at room temperature as I like it sour.