adding submenus to menu

I have a main menu:

<?php wp_nav_menu( array( ‘theme_location’ => ‘primary’) ); ?>

Problem is I don’t know how to make child pages display as submenu items in a menu. I am looking for something like this:

http://guarddoggps.com/

You hover over a menu item, and submenu items display.

Is there a non-plugin way to achieve this in the latest version of wordpress?