drop down menu wont work

drop down menu wont work

so i made a nice menu, and later on i thought, wouldnt it be cool if i
made a drop down menu.
So i started working on it, but it doesnt seem to work.
Link
<div class="nav block">
<ul>
<li style="border-left:1px solid black;" ><a href="#">Home</a></li>
<li><a href="#">About Us</a></li>
<ul>
<li><a href="#"> Porfolio</a></li>
</ul>
<li><a href="#">Reviews</a></li>
<ul>
<li><a href="#">Spellen</a></li>
</ul>
<li><a href="#">Releases</a></li>
<li><a href="#">Contact</a></li>
<li><a href="#">Contact</a></li>
</ul>
</div>