When milk or another liquid curdles, it separates into lumpy solids and watery liquid, usually because something has gone wrong. If you've ever poured milk into your cereal and seen white chunks floating in it, that milk has curdled because it went bad. Hot liquids can also cause curdling: adding lemon juice to hot milk makes it curdle immediately, which is why recipes carefully control when acidic ingredients get added.

Curd is the thick, lumpy part that forms when milk separates. Cheese-makers actually want milk to curdle in a controlled way, using special ingredients to make it happen on purpose. But in most cooking and baking, curdling means something went wrong with your recipe.

People also use curdle to describe how they feel when something is disgusting or disturbing. If a scary story makes your blood curdle, it means the story was so frightening or revolting that it gave you a sick, uncomfortable feeling. When someone says a scream “curdled their blood,” they mean it was bone-chilling and horrible to hear.