When we are retained on a case, we often don't know what our defense is going to be. We have to react to the facts as we find them--and they aren't always all favorable. In this case, we came into an investigation where the client said that he had had a couple of beers, but the breath test said .000/.000. That was good for us, but police took a urine sample, and were claiming that it showed that our client had been smoking marijuana. Knowledge of the pharmacokinetics of marijuana (what happens to the substance once it is taken into the body) allowed us to show the court and the prosecutor that their "positive" urine test for marijuana meant no more than a bus transfer.  The DUI case was dismissed, and the client was happy.