You're on a game show and you have to pick one of three doors for the prize.

You pick door 1.

The host says to open door 2 and of course door 2 does not contain the prize (because the host knows). The host asks you if you'd like to switch your choice from door 1 to door 3 ?? The correct answer is that you should switch to door 3.

The reason is that the host "knew" that one of the two remaining doors (the ones you did not choose) did not contain the prize, so the process was not random and the odds had indeed changed. If the probability of the prize being in door 2 or door 3 is 2/3rds, then given that it's not door 2, then the odds of the prize being in door 3 is 2/3rds.

That's why you cannot win at Roulette ......... sigh