title: Create WPF Watermark in Pure XAML
body: <p>This is for a TextBox control on a login screen, where the TextBox contains the username. I want the TextBox to perform in the following way:</p>
  
  <ul>
  <li><p>When the content is empty the content should be set to "Username".</p></li>
  <li><p>When the TextBox is clicked I want the content to be set to
  "" i.e; nothing (unless the content has already been edited by the user).</p></li>
  </ul>
  
  <p>This is a pretty standard feature nowadays, something like this <a href="http://en.support.wordpress.com/tag/login/" rel="nofollow">wordpress login</a> (at the top of page). <em>coudn't think of a better example than this I'm afraid :)</em></p>
  
  <p>So, anyway, I've already done this using a ViewModel and it works well, but I'd like to know if this can be done <strong>purely from the XAML end</strong>. No business logic is concerned so I think it would be better to do it without the VM.</p>