Checking for DUI

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, he saw a fellow stumble out of the bar, trip on the
curb, and try his keys on five different cars before he found his.
Then, sat in the front seat fumbling around with his keys for several
minutes. Everyone else left the bar and drove off.
Finally, the fellow 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"