I've seen this problem explained in the movie 21, as well as the show Numbers. I'll use the example given in 21. You're on a gameshow, and you're shown 3 doors. Behind one of the doors is a new car, and behind the other 2 are goats. You pick door number 1. The host then opens up door number 3, behind which a goat is revealed. The host then says "do you want to switch to door number 2, or stay with door number 1?" Basically, it is said that it's in your best interest to change your answer to door number 2. When you first picked door 1, you had a 33.3% chance of getting the car, but once door 3 was revealed, door 2 now had a 66.6% chance of it being the car, because of variable change. I have a BSc. in Physics, and this makes no sense to me whatsoever. Here's an argument: you initially pick door 2 in the exact same scenario. By the same logic, you should switch to door 1 after it's revealed that door 3 has a goat behind it. Using the same logic in both scenarios, you would be right in one and wrong in the other. Regardless of what came before, you're still faced with 2 doors in which only one is correct... hence the odds are 50/50. Comments? Corrections?