Hi all, it's me again with yet, another question!

Just spoke to my friend who I am putting a website together for, and although we agreed it would be as basic as possible as my skills are a little rusty, he now wants a drop down menu. Basically I have this:

<ul id="main_menu">
 <li id="quick_contact"><a href="contact_main.html">Contact Us Now</a></li>
 <li><a href="index.html">Home</a></li>
 <li><a href="about.html">About Us</a></li>
 <li><a href="security.html">Security</a></li>
 <li><a href="training.html">Training</a></li>
 <li><a href="testimonials.html">Testimonials</a></li>
And he wants it so when you click on the Security link you get this menu pop up:

<ul id="security_menu">
 <li><a href="c_protection.html">Close Protection</a></li>
 <li><a href="surveillence.html">Surveillence</a></li>
 <li><a href="d_supervisors.html">Door Supervision</a></li>
 <li><a href="e_security.html">Event Security</a></li>
 <li><a href="s_guards.html">Security Guards</a></li>
 <li><a href="r_security.html">Retail Security</a></li>
 <li><a href="bollywood.html">Bollywood Security</a></li>
 <li><a href="filmlocation.html">Film Location Security</a></li>
 <li><a href="cctv.html">CCTV</a></li>
I want it to be as basic as possible and I have done some searching on google but they all include jquery? Which I am not familiar with, if someone could help me out or point me in the right direction that would be great!

I promise soon to stop asking so many questions, honestly!