How to Display First 20 Characters of a Post?

Hi,

Just a quick question here, in my footer I want to display the latest 3 posts, with their post title and 20 characters from their post.

I’ve been able to get the three posts to display with their titles, with (copied this from another site):

<?php
//The Query To Get Posts
$my_query = new WP_Query('showposts=3');

//Run the Query and Loop Through Results
while ($my_query->have_posts()) : $my_query->the_post();
$do_not_duplicate = $post->ID;

?>

//Display Post Title From Each Post
<h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></h2>

/*****Need to display the first 20 characters of the post here!*****/

<?php endwhile; ?>

Please let me know if there is a more efficent method to the above btw, this is just going in my footer.
Anyhow I just need help with getting the first 20 characters of the post to display, I tried using substr like:

<?php
$string = the_content();
$newString = substr($string, 0, 20);
echo $newString;
?>

But this just echoes the content without it being trimmed at all! Can somebody please advise me on a way to do this?

Thanks!