Good news! You don't need to wait until that carton of regular milk in your fridge begins to curdle to take advantage of sour milks charms. Actually, pasteurized milk - which is most of the store-bought kind - isn't that great to use once it sours. For most practical purposes,  you can come up with your own sour milk by taking a cup of store-bought milk and adding a tablespoon of vinegar or lemon juice to it. Let it stand for 5 minutes and then it is ready to use.