Probably by someone leaving milk out to sour, causing it to split into curds and whey. Curds are the most basic form of cheese.
I'm not sure exactly what you're asking, but I'll give a brief example as to how curds are made, and hopefully it will cover your question. Curds are made by curdling milk with an acid like vinegar, lemon juice, or rennet. Raw milk can be curdled by adding lactic acid bacteria, which is then used to make sour milk cheeses. By increasing the acid in milk, it causes the milk proteins to bind to each other, resulting in the curds. When making curds, the liquid that rises to the top, that is drained off is known as whey.
curds and whey is a dairy product made by curdling milk. Curds are solid; whey is liquid.
Check the best before date on it. If milk is kept after the best before date, chances are it's not as good. Also smell it to see if it smells sour or not. If it doesn't smell sour it's still good to drink. The next test is only if the milk itself looks kind of suspicious. Pour a little out on a teaspoon to see if it's consistent and doesn't have curds (or lumps) in it. If there's curds, throw it out.
Assuming you don't keep it in a refridgerator - It will eventually go sour, and separate into the curds (solids) and whey (liquid) components.
The correct phrase is "The sour milk stinks."
Curds are a dairy productobtained by coagulating milk in a process called curdling. Whey is the liquid remaining after milk has been curdled and strained. The whole mess is similar to cottage cheese.
This product is called sour milk. It causes milk for form a coagulate. It is not a precipitate as it doesn't separate into a solid and a liquid. Cheese making would use that method.
"Curds" - when milk is used for cheesemaking, it is separated into curds (the milk solids) and whey (a watery residue) by the addition of an acid, or rennet (an enzyme). The curds are then processed to become many different forms of cheese, but the cheese which most closely resembles curds in it's unprocessed form is "cottage cheese".
sour milk
Sour milk is milk that is off. You drink, you get diarrhoea. So don't drink sour milk.
Both: a physical change as the curds (solids) separates from the whey (liquid), and a chemical change as the decomposition of the milk takes place, producing molds on the surface and a sour smell and taste.