How curd is made?

When some acidic substance like lemon juice or vinegar is added to milk, the milk protein gets tangled into solid masses called curd. The water separated out is known as whey and which is drained off to retain only the curd. This process of milk coagulating to form curd is known as curdling. Curd can also be made by adding lactic acid bacteria to raw or boiled milk. Also, milk that has been left without refrigerating also forms curds.