female mammals which is produced in the milk glands. The constituents of milk depend upon the species of mammals. For example, the human milk is richer in lactose (name of a chemical compound) compared to the milk of a cow which, in turn, has more lipids in it than human milk. But all kinds of milk contain some common constituents, their degrees vary, like water, lactose, carbohydrates, lipids, proteins, dietary minerals etc.
Now, because of its organic nature the presence of some little creatures (micro-organisms) is natural. Normally, they are killed off by heating the milk for a short time and then keeping it in a closed cool place but if it is not done, or if the microorganisms are allowed to remain or introduced from outside, for example, by spilling a couple of drops of lemonade, a chemical reaction takes place and turns the milk sour.
I still didn’t get my answer!
The action is usually carried out by bacteria. They react with lactose and break it up into lactic acid. Lactic acid tastes sour. This change also coagulates a milk protein (casein) in the milk, causing the milk to separate into a watery layer and a thick clumpy substance. That thick clumpy substance is also glue.