Why does drinking milk make your stomach hurt?

Answer 1

Because of the rennet in your stomach.

Rennet is an extract found in the stomachs of all mammals. This extract contains a number of enzymes which are designed to help these animals digest their mother's milk, and when added to milk, rennet will cause the milk to coagulate, forming curds and whey.

Answer 2

Lactase is the enzyme that metabolizes lactose into glucose and galactose. Most mammals stop producing lactase after weaning. An estimated 75% of human adults worldwide show a decrease in lactase production during adulthood, including 5% of northern Europeans, 71% of Sicilians, and more than 90% of some African and Asian populations. Therefore, the majority of adults are unable to fully digest lactose, a sugar found in milk and other dairy products. For them, consuming dairy products causes symptoms such as abdominal bloating and cramps, flatulence, diarrhea, nausea, rumbling stomach, or vomiting, and may be a significant cause of irritable bowel syndrome. People in northern Europe probably retained the ability to produce lactase into adulthood through natural selection. Crop failures during ice ages caused people to turn to the unnatural act of drinking the milk of other animals to survive. The humans who were able to tolerate this without vomiting survived and reproduced, passing on their mutation to their children.

Answer 3

When milk curdles, it turns into cheese. Different types of milk and adding different spices and such to the milk before/after you curdle it gives you different types of cheese.