Manually Inserting Related posts.. Help!

Hi..
I wanted some help.. i wanted to knw how to manually display related posts .. i inserted this following code to my single.php But it was not successfull.. when i tried, the pages that use single.php dint load..

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<?php related_posts(); ?>
<?php endwhile; ?>
<?php endif; ?>

Please tell me where i have gone wrong.. thanks..