Before Jumping into the Hack, There is two other way to acheive the same.

1. Edit the footer.php file in the wordpress theme folder (wp-contents/themes/) to add the Google-ad code. 
   (The disadvantage of this method is that, when you change the theme, you need again edit the footer.php file in the new theme)
2. Using a wordpress plugin post footer. By this plugin, one can add  any text/html to the footer of the posts. 
   (Disadvantage : Plugin Advertisement / unwanted  upgrades)

Now, the Hack Script is as follows
Place your google code at the html commented section of code below

 <?
  function sp_add_page_footer() {
 ?>
 <!--You're text or google Ad goes here--> 
 <?
 }
 add_action('get_footer', 'sp_add_page_footer', 10);
 ?>

 That’s it.. copy the code, place it in my-hacks.php which is created at the wordpress root folder.  Voila!! The first wordpress hack

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