Variable and tween problem

1

below is an example of my code.

! the mc only tweens once. why??? !

the myVar traces correctly on every roll over. but the mc only tweens on the first roll over.

import mx.transitions.Tween;
import mx.transitions.easing.*;

var myVar:Number = (mc._x + 35);

button.onRollOver = function(){
trace (“Roll Over”);
trace (myVar);
var mytween:Tween = new Tween (mc, “_x”, Strong.easeOut, mc._x, myVar, 1, true);
}