Here's the thing, with the guy giving the money away. The point is for me to lose, therefore he keeps the money. Now I've picked A, he shows me B is empty and offer to let me switch doors to C . That tells me I'm 100% right because if I have picked the wrong door why offer it up? However by offering the third option it gives the guy giving the money away a 50/50 shot at keeping the money. Increasing HIS chances, you should never switch. Unless it was established before the game started that one wrong door would be revealed, then I would switch.