title: What exactly are expressions that produce *side effects*?
body: <p>I have trouble understanding this paragraph in the page <a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special/void" rel="nofollow">https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special/void</a>:</p>
  
  <blockquote>
    <p>This operator allows inserting
    expressions that produce side effects
    into places where an expression that
    evaluates to undefined is desired.</p>
  </blockquote>
  
  <p>What exactly are expressions that produce <em>side effects</em> ?</p>