The Riddle That Seems Impossible Even If You Know The Answer

The key is that if you start with the box labeled with your number, then either you arrive at a box that contains that number or you open boxes endlessly. Given that there are a finite number of boxes, that’s impossible. So by starting with the box labeled with your number, you know that you are in the loop with a box that contains your number. The only issue is whether that loop is 50 boxes or fewer.