system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is milk good or bad for you?
gpt
People are not all the same with regard to milk, so when you ask if milk is good or bad for you, it depends who you are talking to.  Furthermore, there is more than one kind of milk.  The most usual kind is cow milk, and that is high in a type of sugar called lactose (which unlike other sugars does not have a sweet taste, but does help to make milk taste milky) and which many adults (and some children) cannot digest.  Inability to digest lactose is called lactose intolerance.  So for a person who is lactose intolerant, it is bad to drink cow milk.  For those who are able to digest lactose, which includes almost all children, milk is a highly nutritious drink, which is a particularly good source of vitamin D and calcium in the diet.  Children in particular are in need of good nutrition in order to grow in a healthy manner.  Children who suffer from a lack of vitamin D will develop a disease called rickets, whose main symptom is weak and unhealthy bones, which break easily.  Of course, there are other possible sources of vitamin D (fish is very good) particularly for adults.  Babies cannot chew food the way adults do, so they have fewer options.  It is also known that the most beneficial form of milk for babies is not cows milk, but their own mother's milk.  That has many benefits, not just vitamin D but also useful antibodies to strengthen immune response, and other useful nutrients.  Breast feeding generally works very well to nourish babies.