Answer:
Suppose you would be hypnotized. Then the person who does the hypnotizing could have you sit in an awkward position for a long time. So the point is that sitting still is not really a problem of the body, but of the mind. The body often reflects your state of mind. So if your mind would be made 'visible' like the body is, then it would seem quite restless. So the real question is: how to comfort and relax the mind? The answer is simple. Meditate. After a while the mind will settle down and then the body will follow.