Makes about 4 cups
Rinse the inside of a medium saucepan with cold water, and then add:
4 cups whole milk (not ultra-pasteurized)
Watching carefully, bring the milk almost to a boil, about 210°f.
Remove from heat and allow milk to cool to warm skin temperature, about 110°f.
½ cup plain yogurt
Pour the mixture into wide-mouth quart glass jars. Screw lids on the jars, then wrap in a towel or put a thick winter hat over them and leave in a warm place, about 100°f, for about 8 hours so the yogurt can ferment and thicken. The jars may be placed in an insulted cooler along with an additional jar filled with boiling water to create a 100°f environment for the yogurt to ferment.
Refrigerate. Homemade yogurt will keep for 1–2 weeks refrigerated.