public function displayScreen(screenClass:Class):void
{
    // if a current screen exists, remove it
    if (currentScreen != null)
    {
        currentScreen.bringOut();
    }

    // if the screenClass we're told to create is the Game class
    if (screenClass == Game)
    {
        // make it and save it as currentScreen
        // (we need this step because the game needs more than just "this")
        currentScreen = game = new Game(levelXML, this);
        addChild(game);
        game.begin();
        game.bringIn();
    } else {
        // otherwise, just make the screen
        // (this will work with everything else)
        currentScreen = new screenClass(this);
        addChild(currentScreen);
        currentScreen.bringIn();
    }
}