title: Sealed must be used with override?
body: <p>From msdn <a href="http://msdn.microsoft.com/en-us/library/88c54tsw.aspx" rel="nofollow">sealed (C# Reference)</a></p>
  
  <blockquote>
    <p>"When applied to a method or property, the sealed modifier must always be used with override."</p>
  </blockquote>
  
  <p>Why must it always be used with override?</p>