Police Officer Staking Out A Bar

One night a police officer was staking out a particularly rowdy bar for possible DUI violations. At closing time, he saw a fellow tumble out of the bar, trip on the curb, and try his keys in five different cars before he found his. 

Then he sat in the front seat fumbling around with his keys for several minutes. Everyone else left the bar and drove off. Finally he started his engine and began to pull away. The police officer was waiting for him. 

He stopped the driver, read him his rights and administered the 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.”