I hope this problem isn't old hat here. I know the answer, but I would be curious to see your explanations. Suppose you are on a game show, where you are presented three curtains. Donkeys reside behind two of the curtains, and the big prize (tickets to Metallica) are behind the third curtain. Suppose you pick curtain #1. Rather than show you what is behind curtain #1, the game show host opens curtain #2 to show you a donkey. Obviously the Metallica tickets must be behind curtain #1 (your original choice) or curtain #3. Before opening curtain #1, the game show host decides to let you change your mind and pick #3 instead if you wish. Should you switch to curtain #3, or stick by your original choice? Or does it matter? And why?