Confusing Function Scope problem

programming flash
1

I have this function on my main timeline:

function Over(){
[LEFT]     this.bg_mc.tween("_alpha", 0, 0.3)
}

[/LEFT]

This is basically a function for a Rollover so it is called on my button like this:


on (rollOver) {
[LEFT]     _root.Over();
}

[/LEFT]

In theory, this should look into my button MC and fade bg_mc. But it doesn’t work. However, if I change my function to this:

[LEFT]


function Over(){
    this.tween("_alpha", 0, 0.3)
}

[/LEFT]

Then it fades the whole button perfectly. Why won’t the function look inside the MC for bg_mc? Is this a scoping problem?
Thanks
Jamie