Putting something Between The Body Sections

Hi there – I’m trying to figure out how to put something (an ad, but could be just a div full of html) inside my posts.

I want it to appear at the point where the <!– more –> code separates the front page part of a story from the rest. In the event of a short post, it wouldn’t show.

I see that the default code from single.php for the entry area looks like this:

<div class="entry">
<?php the_content('<p class="serif">Read the rest of this entry &raquo;</p>'); ?>
BLAH BLAH BLAH
</div>

First of all, since this is the single.php page, why is there that HTML that says “Read the rest of this entry”???? You’re already on the single page right? So what is that and why is it there?

Second – if I wanted to put “Mary Had a Little Lamb” in between the two sections of the post, how would I go about doing it? It looks to me like there’s only one variable that pulls the whole post. But the front page still manages to get only the first part, so there must be a way to split it and stick something in the middle.

How would I go about doing this?

Thanks!!!!