Why do we put vitamin D in our milk?

Early in the 20th century, children who lived in slums with buildings packed close together did not have much outdoor play space or time outdoors at all. This led to an epidemic number of cases of a disease called "rickets", which was essentially soft bones. It was found that the disease could be avoided by such children taking a daily dose of cod liver oil - rich in Vitamin D. Much to the joy of children everywhere, cod liver oil declined in popularity when vitamin D began to be added to milk, instead.