title: onClick not respond for hide() and show() function
body: <p>sorry everyone, i have a question. why function didn't respond?</p>
  
  <p>my html :</p>
  
  <pre><code>&lt;img id="btn" src="img/button.png" onclick="show()"&gt;&lt;/img&gt;
  ...
  &lt;img id="xxx" src=""&gt;&lt;/img&gt;
  </code></pre>
  
  <p>my script :    </p>
  
  <pre><code>function show(){
      document.getElementById("xxx").src = "img/src.png";
      document.getElementById("btn").onclick = "hide()";
  }
  function hide(){
      document.getElementById("xxx").src = "";
      document.getElementById("btn").onclick = "show()";
  }
  </code></pre>
  
  <p>if i click the button, only first function [<code>function show()</code>] which respond. please help. thanks.</p>