title: What does "sending a message to nil" mean, and why is it a special case?
body: <p>I just started reading the Objective-C tutorials, and there is a section on "sending a message to <code>nil</code>":</p>
  
  <blockquote>
    <p>There are several patterns in Cocoa
    that take advantage of this fact. The
    value returned from a message to nil
    may also be valid:</p>
  </blockquote>
  
  <p>What does this mean? I can't seem to follow it.</p>