Menu In Which The Buttons Move Each Other? HELP!

Ok, I tried searching for this as I’m sure it has been asked before but I couldn’t think of the right key words. Anyway, what I’m trying to find out is: How can I make a menu that, when you roll over one button, it expands and kind of pushes the others out of the way?

Here is an example of what I mean:

Hope someone can help.