Prototype questions

I been reading alot of tutorails on prototypes and inhertitance, and trying to understand what there saying. :smiley:
I was wondering, how would i go about assigning a ‘class’ to only 3 movie clips. I am not fully grasping this Obeject.registerClass() command. The ‘id’ of a movie clip is not the instance name? If i have a movie clip on the stage , can i assign a class to it? How do i do that? Does somebody have a simple .fla i can take a gander at?