Mack is talking to his friend Don.
(M = Mack, D = Don)
M : Hi, Don! How are you doing in your English class?
D : Not so well, I'm afraid.
M : What's the problem?
D : I'm not improving. Tell me, how come your English is so good?
M : Well, uh...I have an American girlfriend.
D : Aha! That's it. Now I know what to do. (He runs off.)
M : Hey, come back! I was just kidding!