[KJL]

I didn't read the other posts so I am most likly repeating another poster. When you pick your door you have a 1/3 chance of picking the right door. The host then shows you a door with a O% chance of being right. Basic math tells you the remaining door as a 2/3 chance of being correct.

Another way to look at it is logically. For these examples say that the prize is behind door 3.
If you pick Door 1 the host shows you Door 2, if you switch you will pick the correct door.
If you pick Door 2 the host shows you Door 1, if you switch you will pick the correct door.
If you pick Door 3 the host shows you either door, if you switch you will be wrong.
So if you switch you are correct 2/3 times.
If you do not switch you will only be correct when you pick the right door, which is 1/3 times.