Place 3 tablespoons of organic full-fat yogurt into a clean glass jar.
Pour cold milk directly over the yogurt. No need to heat it.
Then, stir the milk and yogurt until fully combined.
Cover the container loosely with a lid, plastic wrap, or a clean thick towel. Place the glass jars inside your oven with the light turned on (but no heat). The light bulb provides gentle warmth (usually 100–110°F), the right temperature for culturing. Do not turn on the oven itself or you will catch your dish towel on fire and cook your ferment!
Next day (12 to 24 hours later), once the yogurt has thickened, move it to the fridge for at least 4 hours to fully set for best results. It will firm up more as it chills. (The recommended time to ferment the yogurt is between 12 to 24 hours. I highly recommend tasting the yogurt at 12 hours and seeing if you like to tartness. SEE TIPS)
(OPTIONAL) Strain through cheesecloth or a fine mesh strainer overnight to remove the whey for a thick Greek yogurt. Remove the whey but do not throw it away! Use it to make Fermented Strawberry Lemonade or Lacto Fermented Lemonade.