>     Staking out a notoriously rowdy bar for possible D.U.I.
>     violators, a cop  watched from his squad car as a fellow stumbled
>     out the door, tripped on the  curb and tried 45 cars before
>     opening the door to his own and falling asleep on the front seat.
>
>     One by one, the drivers of the other cars drove off. Finally, the
>     sleeper  woke up, started his car and began to leave. The cop
>     pulled him over and  administered a Breathalyzer test. When the
>     results showed a 0.0  blood-alcohol level, the puzzled policeman
>     asked him how that was possible.
>
>     "Easy," was the reply. "Tonight was my turn to be the decoy"

(Thanks to Ben L.)