Sourness is the sharp, tart taste that makes your mouth pucker and your face scrunch up, like when you bite into a lemon or taste vinegar. It's one of the five basic tastes your tongue can detect, along with sweetness, saltiness, bitterness, and umami.

Sourness comes from acids in food. Citrus fruits like lemons, limes, and grapefruits are sour. So are pickles, which get their sourness from vinegar. Yogurt has a gentle sourness, while sour candy is intensely sour on purpose. When milk goes bad, it develops an unpleasant sourness because bacteria have created acid in it.

The word can also describe a person's attitude or mood. When someone speaks with sourness, they sound bitter, resentful, or unpleasant, like they've been dwelling on disappointments. A neighbor might look at your new bike with sourness because they're jealous. Someone whose plans fall through might respond with sourness instead of staying good-humored.

In cooking, chefs use sourness deliberately to balance other flavors. A squeeze of lemon can brighten a rich sauce, and the sourness of tomatoes balances the sweetness in ketchup. Without sourness, many foods would taste flat and one-dimensional.