Although their exact origin remains a mystery, we do know that yeast-based kefir grains have always been at the root of the kefir phenomenon. These grains are capable of a remarkable feat: in contradistinction to most other items you might find in a grocery store, they actually expand and propagate with use. This is because the grains, which are granular to the touch and bear a slight resemblance to cauliflower rosettes, house active cultures that feed on lactose when added to milk. Consequently, a bigger problem for most kefir drinkers is not where to source new kefir grains, but what to do with the ones they already have!