HOW to create expandable/collapsible category menu?

i have some terms with child terms within the vocabulary. What i want to do is navigational menu with only parent terms listed at first, after clicking on one of the parent items and page reloading we see parent category expanded with child terms listed, how to do this? Parent and child items should be added automaticly to m\the menu, not by the hand. I dont want to use category module, it's not a good decision for my situation. Please help...