add_filter( 'the_content', 'our_modify_the_content' );      //the_content gets the post/page content retrieved from the database, prior to printing on the screen
function our_modify_the_content( $content )
{
  //Exit if not a blog post (the_content is also fired by pages)
  if (!is_single())
    return($content);

  $Output = '';
  $Output .= 'Something<br>';
  $Output .= $content;
  $Output .= 'Something else';
  return $Output;
}