Related posts by searching post tags of single post as terms

If you are inside the loop, on the single page, than you have all tags and can search for releated posts with the same tags. You find many solutions via google; an example:

//for use in the loop, list 5 post titles related to first tag on current post
$tags = wp_get_post_tags( get_the_ID() );
if ( $tags ) {

  echo 'Related Posts';

  $first_tag = $tags[0]->term_id;
  $args = array(
    'tag__in'          => array( $first_tag ),
    'post__not_in'     => array( get_the_ID() ),
    'showposts'        => 5,
    'caller_get_posts' => 1
   );

  $my_query = new WP_Query( $args );
  if ( $my_query->have_posts() ) {

    while ($my_query->have_posts()) : $my_query->the_post(); ?>

      <p><a href="https://wordpress.stackexchange.com/questions/51599/<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></p>

      <?php
    endwhile;
  }
}