title: How to select an element by ID using jQuery
body: <p>I am trying to use the following jQuery code to add a div with class <code>col_box</code> inside the <code>col_left</code> div:</p>
  
  <pre><code>$('#col_left').add('div').addClass('col_box');
  </code></pre>
  
  <p>My DOM tree looks like this:</p>
  
  <pre><code>&lt;div id="header"&gt;Header&lt;/div&gt;
      &lt;div class="col_container"&gt;
              &lt;div id="col_left"&gt;
                  &lt;div class="col_box"&gt;A&lt;/div&gt;
                  &lt;div class="col_box"&gt;B&lt;/div&gt;
              &lt;/div&gt;
      &lt;/div&gt;
  </code></pre>
  
  <p>However the jQuery code isn't working. It adds the class <code>col_box</code> to every element on the page.</p>