RE:2 questionsmingzhu, your comment below could not be further from the truth.
Milk is the only natural product which include all essentials baby need to survive, the only food human can survive without any other food.
Babies are to be raised on breast milk. If that is not possible then baby formula is to be used.
Cow milk is intended for baby calfs however people have also started to enjoy it as well. However it does not fully support a babies nutricianal needs.