Probability Changing doors on a game show?

Just watched the movie 21 and there is that one scene where the professor sets up a word problem. He says that there are three doors two of which have goats behind them and one has a car. The student picks door 1 (at this point it is a 1/3 chance of a car). The professor then opens door 3 and says that there is a goat behind it. He then asks whether the student wants to stick with door one or change to door two. The student then decides to switch to door two because he claims that there is a 2/3 chance that the car is behind that door. WHY? When did it change from a 1/3 probability. I mean I understand the concept that door three being opened and revealing a goat changes the chances of the remaining two doors to 50/50 but how does it change the door that the student first picked to a 1/3 chance of being right and the second door into a 2/3 chance of being right. Can someone please explain this?