CSS drop down menu not displaying on the top (not showing on the top)

• use 'z-index'  and position property should be set to 'relative'
• set  'z-index' attribute to a value higher than the other elements

 ul li li {  
      display: block;  
      position: relative;  
      float: left;  
      background-color: #000;  
      z-index: 200;       
 }  

That's it..!