I don't know if this is the right place to ask questions about Google sites. I wonder if it is possible for a Google site to have a horizontal menu on the top, and clicking one item of top menu would display its sub-menu on the side bar? Thanks.
You can do that! You have to add it on the page which you open with the horizontal menu.
Edit page -> Insert -> select sub-page listing.
In the sub-page listing you can choose from where and how many levels you want to show in the menu.
look at this explanation to get an ideia how to do it.