You are playing a game. A person (game host) shows you 3 boxes. 1 box contains £100, 2 others are empty. You need to choose one of the boxes. If the box you chosen contains the money you win.
Suppose you make a guess and point at some box. The game host instead of opening the box you just pointed at opens one of the other boxes and shows you that that box is empty. Then he asks "I'm going to give you a last chance to change your decision". So you can either stand by your original choice and point at the same box or choose the other one. What gives you better chances? (i.e. what are the chances you win if you choose the box you originally selected vs. the other box?)