Buttermilk is one of those tricky ingredients that's always sitting in the fridge in abundance when you don't actually need it. But the second you start whipping up pancakes, that once-extra buttermilk has somehow vanished, nowhere to be found. That's why it's helpful to know a buttermilk substitute or two, for those times when you're convinced that buttermilk's greatest quality is inexplicably disappearing and you're too busy to make it to the store to pick up some more. But it helps to find a good substitute for buttermilk if you can answer the seemingly basic, but important, question: What is buttermilk?

Well, according to the editors at Cook's Illustrated, buttermilk used to be made from "the liquid left behind after cream was churned into butter." That cream was left to "ripen" for a couple days, which would give bacteria a chance to "ferment by converting milk sugars into lactic acid, which made the resulting buttermilk mildly sour and slightly thickened." Because of the rise of pasteurization, however, those bacteria aren't given a chance to form in the same way, so these days, most buttermilk is cultured buttermilk, "made by reintroducing lactic-acid bacteria to pasteurized skim or low-fat milk," rather than letting them occur naturally. As a result, cultured buttermilk, as L.V. Anderson writes in Slate, is thicker in texture and more acidic and sour than its naturally cultured counterpart.

But what all this means, practically, is that buttermilk is regular milk that's basically curdled and sour—so if you're looking for an easy buttermilk substitute, all you have to do is add a tablespoon of either lemon juice or white vinegar to a cup of milk. Let it stand for five to ten minutes, until you see some curds forming. You can use non-fat milk or whole or even heavy cream, whatever you have on hand (though this method does work best with a milk with a higher fat content).

If you're looking for a substitute for buttermilk that's a little thicker than the acid-and-milk option, you can use watered-down yogurt or sour cream. According to Anna Stockwell at Epicurious, mix together about ¼ cup of water with ¾ cup of unsweetened yogurt, or equal parts water and sour cream.

Joy Wilson, a.k.a. Joy the Baker, recommends making a buttermilk substitute by stirring together 1 cup of milk with 1¾ teaspoon cream of tartar, which is also acidic. She also notes that you can make a dairy-free buttermilk substitute by mixing almond milk and almond yogurt with a splash of white vinegar.

Perhaps the easiest buttermilk substitute to make at home in a pinch, however, is powdered buttermilk. The team at Bon Appetit has a soft spot for Saco's powdered buttermilk because, " Unlike a lot of commercial buttermilks out there, which are made with skim or low-fat milks, Saco brand powdered buttermilk is made from real naturally cultured buttermilk, i.e. the liquid that's leftover after you churn cream into butter." That means you're getting more of the original, buttermilk taste without the artificially added bacteria. The best part? Powdered buttermilk is shelf-stable, so you can leave it in your pantry until you need it—and you never have to worry about it mysteriously disappearing.