[CS3 AS2] Clickable area of MC full size

programming flash
1

I know this is basic, but I lack the terms to use to search for the solution. I can do some advanced stuff in Flash, but when it comes to basics like this, I’m at a loss.

I’m trying to have two movieclips call the same function but pass to it a different argument to then process accordingly. What I’ve learned is that even though the MCs are small rectangles on the stage, their clickable area seems to span the entire stage. In this code example, when I click on one MC, the trace gives me both 1 and 2, as if both MCs had been clicked. What am I missing? Attached is the stripped-down FLA.

_root.challenge_mc.onMouseUp = function() {navigateTo(1);};
_root.thought_mc.onMouseUp = function() {navigateTo(2);};

function navigateTo(defNum:Number):Void {
    trace(defNum);
}