text: If on the other hand the car is not behind Door 1 , it is equally likely to be behind Door 2 or Door 3 , and the host is forced to open the only door of this pair which has a goat behind it .
simplified: Then the host , who has knowledge of what is behind every door , opens a different door which he is certain has a goat behind it ( opening either door with equal chances if the car is behind the player 's door ) .