Milk
We're not really supposed to drink milk beyond infancy. The fact that we do, and the fact that we are able to, makes us unique as a species. It's all down to an enzyme called Lactase. This is produced in infant mammals so that they can break down a complex sugar called Lactose which is found in their mother's milk. Without this enzyme, the only way the body can break down this sugar is with bacteria, which creates gas and thus what is commonly known as "the shits". When an animal is weaned off its mother's breast milk, it is a waste of energy to carry on producing this enzyme and so usually the body no longer does so. After all, what kind of species is going to go around, as an adult, sucking milk from another animal's breasts?Well, that'll be us. You see, a genetic mutation occurred in northern European humans a few thousand years ago with the birth of dairy farming, opening the door for humans to exploit a source of sustanance that is rich in fat and protein: other animals' breast milk. Evolution in action, yeah?
Not all humans have the ability to digest lactose. The Chinese can't for example, which makes you wonder whether sales of Immodium in China have increased in parallel with the nation's increased demand for cow's milk. Even if you are of northern European descent this doesn't guarantee that your body will continue to produce lactase for the rest of your life. Adults often develop a lactose intollerance in later life.
I will end this post on milk by noting that "Mammal" means "Animal of the Breast". And yes, as mammals, whales and dolphins breast feed.
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