A cop was sitting in his car

A cop was sitting in his car outside a bar waiting for the drunks to come out. Sure enough a very drunk man soon staggers out. He stumbles around putting his keys into different cars until he finally find his. He gets in and fumbles around some more trying to get the car started. The cop watches him and soon the lot is empty and they guy is still trying to start his car. They guy finally gets the car started so the cop stops him and gives him a Breathalyzer test. It reads 0.0. The cop is stunned and asks they guy how can you be so drunk and get a reading of 0.0. The guy answers "Because I'm not drunk. I'm the designated decoy."