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 ) . Last , the host lets the player choose whether to keep what is behind the first door or to change choices to the third door ( the one the host did not open ) . Although not explicitly stated in this version , solutions are often based on the additional assumptions that the car is initially equally likely to be behind each door and that the host must open a door showing a goat , must uniformly choose which door to open if both hide goats , and must make the offer to switch .