Almost fill a thermos bottle (preferably widemouthed) with milk heated to 100 F.
Add 2 tablespoons of plain yogurt and mix thoroughly.
Put the lid on and wrap the thermos in two or three terry towels.
Set it in a warm, draft-free place overnight.
Pour 1 quart of milk into a casserole dish and add 3 tablespoons of plain yogurt.
Stir well and cover the casserole dish.
Place in a warm (100 F) oven with the heat off. Let it sit overnight.
Mix 1 quart of milk and 3 tablespoons of plain yogurt.
Set an electric heating pad at medium temperature and place it in the bottom of a cardboard box with a lid. (A large shoebox works well.)
Fill small plastic containers with the milk-yogurt mixture and put on the lids.
Wrap a heating pad around the containers, then cover with towels to fill the box and let sit, undisturbed, for 5 to 6 hours.
Pour 1 quart warmed milk into a glass-lidded bowl or casserole dish.
Add 3 tablespoons plain yogurt and cover with the glass lid or a clear glass pie pan.
Place in the sun on a warm (not too hot) summer day and let sit 4 to 5 hours. Watch it to make sure it is not shaded as the sun moves.
Add 1 cup starter to 2 quarts milk and let it sit, loosely covered with a dish towel, on the back of the cooling wood range overnight.
Preheat a crock pot on low for about 15 minutes, until it feels very warm to the fingertips.
Put covered containers of yogurt mixture into the crock pot, cover it, and turn off the heat.
At 35- to 45-minutes intervals, heat the crock pot on low for 10 to 15 minutes.