If anyone has still failed to grasp the fact that switching is the better strategy (2/3 chance of winning) then consider this:

The host Monty has a standard deck of 52 playing cards which are randomly arranged face-down on a table. You win a fabulous prize if you can select the Ace of Spades from among the cards. If you pick any other card, you get nothing.

Now, Monty KNOWS WHERE THE ACE OF SPADES IS before he presents you with your choice. You can choose any of the 52 cards. You make a choice, and the chances you picked the Ace of Spades are 1 in 52 - very slim.

After you've made your choice Monty will do one of two things: In the 1 of 52 chances you were right, Monty will randomly turn over 50 of the losing cards and ask if you want to switch. MOST OFTEN, however, in 51 OF THE 52 CHANCES WHERE YOU WERE INITIALLY WRONG IN YOUR CHOICE, Monty will not simply turn over 50 random cards, but HE WILL TURN OVER ALL BUT THE WINNING CARD.

THUS, in 51 of the 52 possible initial choices you make, an ALWAYS SWITCH STRATEGY will lead to WINNING THE PRIZE, while in only 1 of the 52 possible first choices will an ALWAYS SWITCH STRATEGY lead to a loss.

CONVERSELY, an ALWAYS STAY (with your first choice) STRATEGY will lead to defeat 51 out of 52 times and will lead to a win only once out of 52 times.

Which strategy do you choose? As you can see, most of the time the winning card will be the one card Monty chose to leave face-down and not the card you initially chose.

The same logic applies to the three doors.