title: Get element by id from existing jQuery object
body: <p>Ok this has got to be a stupid simple one, but I just can't find the documentation</p>
  
  <pre><code>  &lt;ul&gt;
        &lt;li class="foo" id="jack"  &gt;&lt;/li&gt;
        &lt;li class="foo" id="jill"  &gt;&lt;/li&gt;
        &lt;li class="foo" id="tom"   &gt;&lt;/li&gt;
        &lt;li class="foo" id="dick"  &gt;&lt;/li&gt;
        &lt;li class="foo" id="harry" &gt;&lt;/li&gt;
    &lt;/ul&gt;
  </code></pre>
  
  <p>And the simple jQuery</p>
  
  <pre><code> var listItems = $('li.foo');
  </code></pre>
  
  <p>now how do I get #jack from <code>listItems</code> ? I already know <code>.eq() .get() .find()</code> ect. but not one that will work in one move. Not based on index, not looking for a child or parent, but based on the id of existing elements selected. Anyone know this one?</p>