In a heavy-bottom pot, scald milk, bringing to just about boiling/bubbling. Watch carefully so it doesn't boil over. Set off heat and cool just till you can hold your finger (clean) in for 10 seconds. Still pretty warm! Stir in 1/2 cup plain unflavored yogurt. Cover and place in a cozy spot, wrapped in a blanket or two. Do not disturb for at least 4 hours or overnight. Place in refrigerator carefully (don't jiggle it at all) and allow to cool before using, or it won't set firm. After it completely cools, you can transfer into another container. It keeps for a week in the refrigerator. To make more, save out a half cup and repeat recipe before yogurt gets sour.