Movieclips

Hi there

I have a question please. I would like to know how does a movie clip know that it has to stop/fadeout etc and new movie clip should load.

for eg

I have 3 buttons&movie clips. Button1 has movieclip1 attached to it, Button2 has movieclip2 attached to it , Button has movieclip3 attached to it.

What I would like to know is that say, I press button1 and movieclip1 starts playing and then somebody clicks on button3 how does movieclip1 that it needs to stop and movieclip3 needs to start playing! (I know I could put a close button on each movie clip so users can close each clip before they press another button)

Theres plenty of examples but here is one to show you what I mean

Thank you for your help…

Is it done through having one empty movie clip and loading external swf files in it?,But I would vale any suggestions…

cheers
yogesh