There are several yogurt-making devices, but you can easily make it at home in a large thermos bottle using a candy thermometer. Sterilize all equipment in boiling water before using. Bring milk to a boil, then cool to 40ºC . In a small bowl blend yogurt with quarter cup of warm milk. Whisk it back into warm milk. Pour into a pre-warmed thermos, seal, and set aside in a warm place for 7 hours. Turn yogurt out into a bowl set in a bowl of ice water, stirring to quicken cooling. Cover and refrigerate for 4 hours, or until thickened. Sweeten to taste, if desired, and serve with raspberries. Yogurt will keep for 4 to 5 days, covered and refrigerated.