title: how to generate AuthenticityToken on rails
body: <p>I build the form tag by myself and when I post the form to server it give me a <code>InvalidAuthenticityToken</code> error, so I want to know how to add it in my own in current situation:</p>
  
  <pre><code>&lt;form accept-charset="UTF-8" action="/crops/update" method="post"&gt;
    &lt;input id="crop_x" name="crop_x" size="30" type="text" /&gt;&lt;br /&gt;
      &lt;input id="crop_y" name="crop_y" size="30" type="text" /&gt;&lt;br /&gt;
    &lt;input id="crop_w" name="crop_w" size="30" type="text" /&gt;&lt;br /&gt;
     &lt;input id="crop_h" name="crop_h" size="30" type="text" /&gt;&lt;br /&gt;
    &lt;input id="crop" name="crop" type="submit" value="Crop!" /&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Response error is:</p>
  
  <pre><code>ActionController::InvalidAuthenticityToken in CropsController#update 
  ActionController::InvalidAuthenticityToken
  Rails.root: /home/mlzboy/my/crop2
  Application Trace | Framework Trace | Full Trace
  </code></pre>