[jQuery] Fade in a div, push the div below down

Hi.

I want to make one div appear by the click on a link, and the div
below the appeared one has to be pushed down some pixels.
So if I click the link inside the top div:

<div id="header">
<a>Show new div</a>
</div>

The div below:

<div id="footer">
</div>

Gets pushed down maybe 100px. Its like changing the margin-top: 0px;
to 100px on the div: footer.

Any plugin or help out there?

Thanks in advance !