Very simple question!

I am very new to AS3, so I cannot for the life of me figure out how to return a value out of a ROLL_OVER function. It seems extremely easy, yet I cannot get it to work. Here is a simplified version of what I am trying to do:

test_mc.addEventListener(MouseEvent.ROLL_OVER, testOver);

function testOver(event:MouseEvent):String
return “hello”;