Small game like drag and drop

Question

i'm new to flash action scrip but i want to create small game like drag and drop. but this has special area to drop and after drop selected object ,then i want to change that object(replace object by another object) using mouse move(scrolling). here what i did up to now
bold area is the specialy i want to know, please help....
ball_mc.addEventListener(MouseEvent.MOUSE_DOWN, pickUp);
ball_mc.addEventListener(MouseEvent.MOUSE_UP, dropIt);
square_mc.addEventListener(MouseEvent.MOUSE_DOWN, pickUp);
square_mc.addEventListener(MouseEvent.MOUSE_UP, dropIt);
function pickUp(event:MouseEvent):void
    event.target.startDrag(false);
    event.target.parent.addChild(event.target);
function dropIt(event:MouseEvent):void
    event.target.stopDrag();
ball_mc.buttonMode = true;
square_mc.buttonMode = true;

Answer 1

I don't know which line you intended be bold, but the following line makes little sense...
event.target.parent.addChild(event.target);
If the parent of the event target is the parent, then the event target is already the child of the parent.