It is not necessarily means that it is friction less,it does have also a friction happening inside its just that we didn't feel it or heard it as some other outside materials do,it is because our bones were covered with cartilages on the point where it have joints and it also have some glands that helps on lubricating it which our body continue to produce for our own consume,but when we're having a vitamin deficiency or on wrong diet then there will be the start of ailment or pain that we'll feel on our joints that the part of our we body move is aching is the time that we feel the bone friction inside,and we'll start to take a medicine for that particular pain that we have...
