Someone on Twitter just posted that the cardboard flap on the top of the box has an actual functional purpose. You're supposed to fold it back . . . and use it as a place to put your ketchup.
So is that the ACTUAL secret, built-in purpose for it . . . or is this just someone's hack? As of now, McDonald's hasn't weighed in, so we don't know . . . but we DO know that the trick really does work.