I went to a psychiatrist because I was having severe problems with my
sex life.  The psychiatrist asked me a lot of questions, but didn't seem
to be getting a clear picture of my problems.
Finally he asked, "Do you ever watch your girlfriend's face while you're
having sex?""Well, yes, I did once.""Well, how did she look?"
"Oh boy,... she looked VERY angry!"
At this point the psychiatrist felt that he was really getting somewhere
and he said, "Well that's very interesting, we must look into this
further.  Now tell me, you say that you have only seen your girlfriend's
face once during sex; that seems somewhat unusual; how did it occur that
you saw her face that time?"
"She was watching her sister and I through the window."