title: Objective-C : Something that I don't understand with subclassing
body: <p>I don't understand something with subclassing in Objective-C : for example, we say that NSString inherits from NSObject, but does NSObject also inherits from NSString ?
  I am wondering this because when I create a new class that inherits from NSObject, I have also access to NSString even if my class doesn't inherits from NSString.
  For example, I can write this in my class that inherits form NSObject :</p>
  
  <pre><code>[NSString stringWithFormat:@"Hello world !"];
  </code></pre>
  
  <p>And what I don't understand is that I can do this even if NSString is a subclass of NSObject and not it's superclass ! !</p>