It's a genuinely unwary visitor who drinks the milk in a bachelor's fridge without first taking a cautious sniff to see if it has gone "off." Several families of bacteria are just as enthusiastic about milk's nutritional value as humans are, and most reveal their presence by excreting lactic acid. It turns the milk sour and unpalatable, but that doesn't mean it's unusable. The baking soda in many recipes requires soured milk to trigger its leavening power, and if you don't have the genuine article on hand, you'll need to produce it artificially.
Measure the quantity of milk called for in your recipe.
Heat the milk in your microwave until it's just slightly warm, or leave it on your counter for an hour or two ahead of time so it comes to room temperature. This step is optional, but the milk will sour more quickly if it's warmer.
Stir in 1 tablespoon of lemon juice or vinegar, or half that amount of cream of tartar, for every cup of soured milk your recipe requires. If you're using the cream of tartar, a dry acid, whisk it for several seconds with a fork to ensure that it's thoroughly dissolved.
Rest the milk on your counter for 3 to 5 minutes, until it begins to separate or "clabber." This is normal, and indicates that the milk's casein proteins have begun to curdle and the milk is ready to use.
Combine the now-soured milk with your remaining wet ingredients or stir it into your dry ingredients, as directed in the recipe.
If your recipe contains more than a half-teaspoon of soda, you might need to increase the quantity of acidity in the milk. If you've made the recipe previously and detected baking soda's distinctively bitter, soapy taste, that's a giveaway. Add another half-teaspoon of acid each time you make a batch, until the soda taste vanishes, and then use that amount in future. If you find the finished baked goods aren't as browned as they should be, you've used too much and should cut back slightly.
Plain white vinegar, apple cider vinegar and mild-flavored white wine or rice vinegar are all appropriate options. If it's compatible with the flavor of your baked goods, more adventurous choices such as raspberry-flavored vinegar can make an interesting variation.
You can also use milk soured this way as a substitute for buttermilk in recipes. It will provide an equivalent degree of leavening, though the end result will lack buttermilk's distinctive flavor. If you only use buttermilk infrequently, consider keeping dry buttermilk powder in your cupboard as an alternative. A quarter-cup of buttermilk powder incorporated into most recipes, along with the soured milk, provides a very satisfactory substitute.