One night, a police officer was staking out a particularly rowdy bar for possible violations of the driving-under-the- influence laws.
At closing time, the officer saw a fellow stumble out of the bar, trip on the curb, and try his keys on five different cars before he found his own. Then he sat in the front seat fumbling around with his keys for several minutes. It took him so long that, even though he was almost the first to leave the bar that night, he was nearly the last patron out of the bar's parking lot! Finally he was able to start his car, and he began to pull away.
The police officer was waiting. He followed the car for a while (tracking it at a safe distance using a fancy tracking device). Then he pulled the driver over, read him his rights, and administered a Breathalyzer test.
The results showed a reading of 0.0.
The puzzled officer demanded to know how that could be.
The driver replied, "Tonight I'm the designated decoy."