I spray a foam stopper with Star San and insert it in the opening of a sanitized fermenter to keep any microbes in the air from getting in during the time between sanitizing the fermenter and transferring wort into it. I also use a sanitized foam stopper in place of an airlock for the first stages of fermentation to avoid suck-back of liquid from a conventional airlock back into the fermenter as the yeast takes up oxygen and wort continues to cool. Once fermentation is going, I replace the foam stopper with a drilled rubber stopper and airlock so I can monitor evolution of carbon dioxide during the rest of fermentation.