My method of explaining it is that you can think of it as getting to pick 1 of 3 doors or 2 of the 3 doors... your original pick has a 1/3 chance... 2/3 of the time it will be one of the other two... if it was not the door you picked, it HAS to be the one that you didn't pick and wasn't opened... meaning the non-picked, non-opened door has a 2/3 chance.