3 1/2 cups milk
1/2 cup buttermilk, room temperature

Mix ingredients and let sit out in warm spot for 24 hours. After that, you have a fresh batch of buttermilk! This works great. Don't forget to save a half cup at the end so you can make a new batch. It keeps going and going! Makes 1 quart.

You can use any kind of milk including dry milk, but regular milk works better than skim or dry milk. This is a good way to use sour milk without throwing it away.