monty hall problem

i watched the movie '21' a couple of months ago and was a bit confused by the theory of variable change as explained by mr. spacey and crew. in case you missed the movie, here's the basic premise:

'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, 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?'

you should indeed change to door no. 2. it's statistically to your advantage.

you buy it?