In front of you there are three boxes. One of these three boxes contain a treasure and the other two are empty.
[___][___][___]Imagine the following scenario. You choose the left box and the man holding the lottery says the following: Before I open this box I will tell you that the middle box does not contain the treasure. Do you still want to open the left box?
Would you go for the left box or you go for the right box?
I remember me and couple of my workmated had a 3 hour fight over that one, Sardotjen. I would definetly change the box since that would boost my chance to win up to 66%. For example, if you have 5 boxes and you choose one. Then the lottery man opens all the empty boxes exept one and asks you if you want to switch. Naturally you switch because the chance that you got the box right on first time is 20%. So changing the box will boost your chance to win up to 80%.
Same rule applies to 3, 4, 5.. 100 boxes.
Source is here, in perl