Jean, that's right - the natural (wild) yeasts in the flour (and to a lesser extent in the air) are activated by warmth and liquid, and they gradually colonize and permeate the starter medium. And there are also natural bacterias in the flour (and in the air) that help. It's possible for a "bad" bacteria to take up residence; but you should be able to recognize it if that happens. The starter will turn pinkish in color, and have an unpleasant odor, rather than a sharply acidic/vinegary/floury smell. Hope this helps - good luck! PJH

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