Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1 [but the door is not opened], and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?
Monty Hall Problem - Numberphile Published on May 22, 2014 Extended math version: http://youtu.be/ugbWqWCcxrg?t=2m32s This video features Lisa Goldberg, an adjunct professor in the Department of Statistics at University of California, Berkeley.