You start off with a chance of 1/3 of getting it right but then after the first door is opened, if you switch, then you double your chances? 2/3? Now i've written it down it doesn't seem right at all but it must be because it can't be a 50/50 chance?
If it's a 50/50 chance then screw you, damn if it's that easy!
I should point out i seem to remember seeing something like this before (might not be this at all) i didn't get it to begin with but kinda do now.
*hovers over the post button for 5mins*