CamiSu, I think Mrrzy's problem is like the one I linked to, you DO NOT want to open the door with an ass behind it.

  prize           empty cup         empty cup
  desired         donkey            donkey    

In the other thread it helped me to understand the "history" of the events. Also, and I assume Mrrzy's problem is the same, the "house" or MC has prior knowledge of where the prize is, and the "house" or MC will always select (from the remaining two) one which does not contain the prize. There is more here than meets the eye on first glance. (It helps to have several explanations.) Any doubters can just do trial and error 1000 times to be convinced of the answer! But make sure you understand the rules.