Zip code
 

A blonde says to her psychiatrist, "I'm on the road a lot, and my clients are complaining that they can never reach me."
He replies, "Don't you have a phone in your car?"
She says, "That was a little too expensive, so I did the next best thing. I put a mailbox in my car."
The psychiatrist, laughing to himself, asks, "Uhh... how's that working?"
The blonde replies, "Actually, I haven't gotten any letters yet."
He asks, "And why do you think that is?"
The blonde in all her wisdom replies, "I figure it's because when I'm driving around my zip code keeps changing."