Dynamic tooltip text

I created some dynamic boxes using the
_root.attachMovie(“box”, “box”+x, x);
Now, I was trying to make the tooltip using the onrollover function for all the boxes created.
I tried using

in the for loop
but it dint work. It says undefined always in the output
Can anyone please tell me how do I get the tooltip to work with dynamic controls.
I want the text corresponding to the particular control/button where the mouse is being kept