OK, I've gotten a couple of emails, so I'll post mom's yogurt recipe. I've watched pretty carefully and I know how she does it.
2 quarts of milk (she uses non-fat)
1/4 c instant dry milk powder (makes it thick and creamy)
Mix them together and heat them in a BIG pot (milk that starts to boil expands and goes over the edge FAST! be sure you stay out of the splash zone!) Heat to 180 degrees, or until the skin forms.
Cool to between 90 and 115 degrees, mix in about 1/4 cup of unflavored yogurt. (check the label, choose yogurt with active cultures, avoid pectin.)
Pour it out into containers, cover loosely, put in oven with just the light on. Cuddle a kitchen towel around them to help hold the heat in.
Wait 3-12 hours, take out of the oven, tighten lids, put in the fridge. After 3 hours it won't get thicker, just tangier. We like it best at 3 hours.
Don't forget to start a new batch before you use it all up!
Sometimes Mom puts coffee filters in a sieve, rests it over a bowl, and puts the yogurt in the sieve to drain (in the fridge). This separates out the whey and makes it thick and creamy - perfect for making spreads, dips, or MY favorite - frozen yogurt!!
Don't throw the whey away, use it in place of plain water for cooking grains (rice, oatmeal etc) or for liquid in soups or other cooking. The water-soluble protein is in the whey, so worst case, use it to water your plants. Don't waste it down the drain.... I like to drink it too - but not at night because it makes me pee a lot and I hate to keep getting up when I've got a good snooze going, you know? Sometimes Mom freezes it into ice cubes and uses it in her breakfast blender drinks.