There are a couple of reasons why milk can curdle. It obviously curdles when it sours and goes bad. Milk and egg products can also curdle when they are added to a hot pot and the temperature is too...
First you need to understand what milk is, other than a just a white liquid you get from mammary glands. Milk is an emulsion of fat, water, lactose (a type of sugar) and a mixture of proteins (loads...