It's a bit too much code to put in here directly, but what it comes down to is, at minimum, you'll need some hefty CSS for this. Using javascript and jQuery can lighten that, so I'll include links to both methods.
HTML + CSS Only
You can use the source on this page to copy an example of how you can do exactly what you're asking via CSS and HTML. This is reported as working in pretty much all current browsers (Opera 7.x + (All Platforms), Mozilla 1.x + (All Platforms), IE 6.x + (Windows), Safari 1.x + (MacOS), Konqueror 3.x + (Linux / BSD)), but if you have to go back to IE 5.x, it starts to fail.
Javascript/jQuery
If you decide that you're open to including Javascript and jQuery, there's a second option that looks a bit simpler to implement: this blog article shows how.
