Heat milk to 185ºF and cool to 105ºF.
Place yogurt in a jar and cover.
Mix cooled milk with yogurt in the jar.
Place jar in a crockpot filled with water on warm setting for 6 hours.
Check if yogurt is set, then refrigerate to cool.
To make Greek yogurt, strain yogurt in a cheesecloth-lined colander for a few hours in the refrigerator.
Discard or save the whey, and store the Greek yogurt.
