Never drink and drive

One night, a police officer was staking out a particularly rowdy bar for
potential violators of the
driving-under-the-influence law. At closing time, the officer notices a patron
stumble out of the bar, trip on the curb and try his keys on six different cars
before finally finding his own. The fellow then opens his car door and proceeds
to sit in the front seat.

By now, everyone else has left the bar and driven off.

After some trouble, the fellow starts the engine and begins to pull away. The
police officer stops the driver in his tracks, reads him his rights and
administers a Breathalyzer test. Amazingly, though, the results show a reading
of 0.00. The puzzled officer demands to know how this could be. I don't get it.
You stumbled out of the bar, tripped on the curb and tried your keys on six
different cars before finally found your own. How can it be that you blow
0.00?

Replies the driver, Fooled you, sir... I'm just the designated
decoy.