I usually explain it to people like this: The Host knows which box has the prize. You choose box A, leaving B and C. The Host opens box B and, re

I usually explain it to people like this: The Host knows which box has the prize. You choose box A, leaving B and C. The Host opens box B and, revealing it empty, gives you information about box C that you don't have about your box since the Host cannot choose your box and must choose an empty box from an information perspective, box C has better odds.