Why doesn't .not() work in this case?

I have been desperately trying to figure out why .not() will not work in this case

link: http://ift.tt/2d3DIwI

Thank you for your help.

$('#site').not('#box')
  .fadeIn();

#site {
  display: none;
}
div {
  border: 2px solid black;
  width: 300px;
  height: 300px;
}
#box {
  background: gray;
}

<script src="http://ift.tt/1oMJErh"></script>

<div id="site">
  <div id="box"></div>
  <div></div>
</div>