A gameshow host shows you a wedding ring, and places it inside a ring box. he has two other ring boxes that he shows you are empty. So you have three ring boxes, one contains a ring and the other two are empty. He mixes these up while you are not looking and labels them A, B and C.
He asks you to select a box.
You pick a box, and the host (himself knowing what resides in each box) opens one of the other boxes to reveal an empty box.
He now asks you whether you would like to stick with your original choice, or whether you would like to swap to the other remaining box.
What should you do?