For the homemade yogurt, bring the milk to a boil. Cool to 35 degrees. Check with a thermometer.
Stir in yogurt, pour into clean screw-top jars and seal tightly. Let rest in oven at 40 degrees for 8 hours.
If oven cannot be set to 40 degrees, then place in a cooler bag. After 8 hours, the yogurt is ready and you can enjoy it or refine it with fruits, nuts, grated chocolate, etc.
The yogurt can be kept in the refrigerator for a few days.