How to make primary menu tab go no where

I have a primary menu tab "service" with other few sub tab that go along. How can that make this primary menu tab "service" go no where when I click on it, but instead it'll list out all other sub tabs that go along it? For example, if I point the mouse to the tab "service" then it should list out all other sub tabs that go along it. But if I click on it, then it go no where. How can I do this?

thanks
chad