title: Javascript click event handler - how do I get the reference to the clicked item?
body: <p>My HTML:</p>
  
  <pre><code>&lt;div id="x" onclick="clickHandler(event)"&gt;
     &lt;div id="button1"&gt;This turns green&lt;/div&gt;
     &lt;div id="button2"&gt;This turns blue&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>So first of all, why am I supposed to be passing "event" into the click handler and is event some kind of system keyword? 
  Also, since the click handler is identified on the container div, how do I know which button has been clicked?</p>