Sour describes a sharp, tangy taste that makes your mouth pucker, like biting into a lemon or tasting vinegar. It's one of the basic tastes your tongue can detect, along with sweet, salty, bitter, and umami. Foods like lime juice, green apples, pickles, and yogurt all have that distinctive sour flavor.

The word also describes something that has gone bad. When milk turns sour, it develops an unpleasant taste and smell because bacteria have spoiled it. Fruit that sits too long can ferment and taste sour instead of fresh and sweet.

Beyond food, sour describes a negative or unpleasant attitude. If someone has a sour expression on their face, they look grumpy or disapproving. When a person's mood sours, it means they've become irritable or unhappy. A sourpuss is someone who seems chronically grumpy. You might notice a friendship go sour when people who once got along well start arguing or avoiding each other.

The phrase sour grapes comes from an Aesop's fable about a fox who can't reach some grapes and convinces himself they probably taste bad anyway. When someone dismisses something they wanted but couldn't get, claiming it wasn't worthwhile after all, that's sour grapes.