A blonde is speaking to her psychiatrist. 
"I'm on the road a lot, and my clients are
complaining that they can never reach me." 
Psychiatrist: "Don't you have a phone in your car?" 
Blonde: "That was a little too expensive, so I did
the next best thing. I put a mailbox in my car." 
Psychiatrist: "Uh ... How's that working?" 
Blonde: "Actually, I haven't gotten any letters yet." 
Psychiatrist: "And why do you think that is?" 
Blonde: "I figure it's because when I'm driving
around, my zip code keeps changing."