Monty Hall problem
The key is the bold word:
Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?
The game show host is using his/her knowledge of the answers to pick which door to show you. That imparts to you additional information, which you can use to boost your chances.
The example posed in this thread is contrived, and eliminates 1/3 of the cases before you get to decide whether to change doors:
You pick door #2. You are then shown door #1, whether it's good or bad. If it's good (that's 1/3 of the time), you lose. If it's bad (that's 2/3 of the time), you can switch if you want, but it's 50-50 either way. Overall, your chances of winning are therefore ( 1/3 x 0 ) + ( 2/3 x 1/2 ) = 1/3 with either strategy.
In the Monty Hall problem, the rules are slightly different, because all three cases are considered:
You pick any door, but without loss of generality we can assume it's door #2. You are then shown another door that is known to have a goat behind it. (This is always possible, because the host gets to peek.) In the case where door #1 is a winner, the host will show you door #3 instead of door #1, so you will win by switching doors. It's no longer 50-50. If door #1 is not a winner, it's still 50-50 no matter which strategy you pick (the first case above). Overall, your chances of winning by staying with door #2 are therefore ( 1/3 x 0 ) + ( 1/3 x 1 ) + ( 1/3 x 0 ) = 1/3 while your chances if you change doors are ( 1/3 x 1 ) + ( 1/3 x 0 ) + ( 1/3 x 1 ) = 2/3, i.e., changing doors is better.
In a way, it's like this game: I think of a color (red, green, or blue). You guess what I'm thinking. If you get it right, you win. If you guess the color wrong, we play a second round where I think of an animal (dog or cat) and you guess which one. If you get it right, you win. If you get it wrong, I win. In this thread we're talking only about your chance in the second round, clearly 50-50. In the Monty Hall problem, we're talking about the overall game, where you win 2/3 of the time, either by guessing the color or by guessing the animal.