Struggling with "the content"

Hi guys

I have some custom post types, which I’m trying to pull out the first x no of words / letters of the post’s content.

However, when I use for example:

substr(the_content(),0,100);

this returns the entire post…

I’ve tried using:

$id->post_content

but this returns nothing…

I’ve also tried:

<?php $cs_content = the_content(); ?>
<p><?php substr($cs_content,0,10); ?>

But again this returns the entire post..

Any help would be MUCH appreciated.. This has been going on for a couple of days..

Thanks