Drop-down categories in primary menu

Hello, The primary menu at the top of my page (http://theholyfish.com/) has some categories, e.g. 'Travel'. Is it possible to have the menu show the hierarchy of categories via a drop-down menu? For example, if a user hovers over 'Travel' then a drop-down would show the sub-categories like 'Travel Tips', 'Travel Destinations' etc. Apologies if the solution is something obvious that I've missed. Thanks, Mat.