Heat milk to 104 F (40 C). No termomether? No problem. At the right temperature it will feel hot to the touch of the hand and slightly warm to the touch of the lips.
Then we mix in 6 oz (180 g) of store bought yogurt and place the mixture in a very clean jar.
Heat the oven at a very low temperature for about 5 minutes.
Turn it off and place the jar in the oven on top of a baking sheet.
Leave it there for 8 to 10 hours, or overnight.
Greek Yogurt
Let yogurt rest over a sieve lined with cheesecloth (or any other fine mesh) in the fridge to thicken - at least six hours.
Notes
It will keep up to 10 days in the fridge (if closed) and for about 5 days after opening.
Alternative version:
In a 2,5 quart glass jar mix store bought yogurt and 2,5 cups milk.
Set aside.Add 1,5 cup milk to a large microwavable cup and heat it on high for 2 minutes and 30 seconds (it should boil).
While stirring, add hot milk to yogurt and milk mixture.
Mix well and place jar in warm place (oven) for 8 to 10 hours.