what code do you need to allow people to drop and drag with mouse in Flash

heres the code i copied off the web for dragging a set of blocks into their compartments.

Is "this.startDrag(true)" a generic function used to enable the mouse to pick up objects and does this function need a header file??
could ye explain a few other lines of the code and new functions.


var counter:Number = 0;
var woodSound:Sound = new Sound();
woodSound.attachSound("woodblock");

onEnterFrame = function():Void{
if(counter == 4){
reply_txt.text = "Congrats, you're finished!";
}
};


square_mc.onPress = function():Void {
this.startDrag(true);
reply_txt.text = "";
this.swapDepths(this.getNextHighestDepth());
xstart = this._x;
ystart = this._y;
};
square_mc.onRelease = function():Void {
this.stopDrag();
if (eval(this._droptarget) == squareTarget_mc) {
reply_txt.text="You got it!";
this.enabled = false;
counter++;
woodSound.start(0,1);
}
else{
reply_txt.text="No, keep trying!";
this._x = xstart;
this._y = ystart;
}
};


flower_mc.onPress = function():Void {
this.startDrag(true);
reply_txt.text = "";
this.swapDepths(this.getNextHighestDepth());
xstart = this._x;
ystart = this._y;
};
flower_mc.onRelease = function():Void {
this.stopDrag();
if (eval(this._droptarget) == flowerTarget_mc) {
reply_txt.text="Yes, that’s right!";
this.enabled = false;
counter++;
woodSound.start(0,1);
}
else{
reply_txt.text="Try again!";
this._x = xstart;
this._y = ystart;
}
};


triangle_mc.onPress = function():Void {
this.startDrag(true);
reply_txt.text = "";
this.swapDepths(this.getNextHighestDepth());
xstart = this._x;
ystart = this._y;
};
triangle_mc.onRelease = function():Void {
this.stopDrag();
if (eval(this._droptarget) == triangleTarget_mc) {
reply_txt.text="Way to go!";
this.enabled = false;
counter++;
woodSound.start(0,1);
}
else{
reply_txt.text="Sorry, try again!";
this._x = xstart;
this._y = ystart;
}
};


pentagon_mc.onPress = function():Void {
this.startDrag(true);
reply_txt.text = "";
this.swapDepths(this.getNextHighestDepth());
xstart = this._x;
ystart = this._y;
};
pentagon_mc.onRelease = function():Void {
this.stopDrag();
if (eval(this._droptarget) == pentagonTarget_mc) {
reply_txt.text="Wow, great job!";
this.enabled = false;
counter++;
woodSound.start(0,1);
}
else{
reply_txt.text="Don't give up yet!";
this._x = xstart;
this._y = ystart;
}
};

i got a book on actionscript and i am reading it now. I tried all of the above from the website tutorial and i can pick up the object but it wont put it down when i release the object. see my other threads.

It must be something fairly straightforward which i am missing..