Q: A few months after he got his driver's license, we found a bag of marijuana in our 16-year-old son's car. He swore it wasn't his and that he didn't know whose it was and that he's never used pot, but then he failed to pass a drug test. Now he admits that he's smoked pot, but he says he only tried it once and didn't like it. He still maintains that the pot we found in the glove box of his car belonged to someone else, but he still maintains that he doesn't know who. We think he's lying to us, but we don't know what to do. Pot is legal in our state now, which only contributes to our dilemma. Can you help us out?

A: In my experienced estimation, the likelihood that your son is lying to you is somewhere in the general vicinity of 99.9999 percent. Someone riding in the front passenger seat of his car put a bag of marijuana in the glove box and he doesn't know who the person was? C'mon! And he's only smoked once and didn't like it and he hasn't smoked since but failed a drug test? Double c'mon!

You’re in a dilemma in part because marijuana is now legal in your state? Excuse me, but marijuana is not legal for 16-year-olds, and marijuana obtained illicitly is not legal for anyone of any age, one reason being that illicit sellers often adulterate the pot they sell with various drugs, including methamphetamine.

In my public presentations, I often say that proper parenting is an act of love for one’s neighbor. In other words, one’s responsibilities as a parent do not begin and end with one’s child. In this case, you have a responsibility to the citizens of your community — your neighbors, in the broad sense of the term — to make sure that your son is not a danger to others while he’s behind the wheel of a car.

How do you discharge that responsibility? By lying to you, your son has given you no rational choice other than to confiscate his car for anything other than driving to and from work (if he has a job). What about school? Can you say “the bus”? You certainly should not be inconvenienced because of his irresponsibility. Besides, suffering the humiliation of riding the bus will serve as a daily reminder of his delinquency.

You want to do something here that will, in the parenting vernacular of the 1950s, get through his thick skull. In that regard, I recommend nothing less than a one-year impoundment during which time he is drug-tested randomly but no less than once every four weeks. If he fails a test, sell the car. You might — and I stress “might” — consider letting him reduce his sentence to nine months by getting straight A’s in school, no pun intended.

As for the “friend” who oh-so accidentally left pot in your son’s car, I’d tell him that since he doesn’t know who did it, you will simply have to let all his friend’s parents know that one of their kids is a pot smoker and perhaps even a dealer. Having once been a teenage boy myself, I virtually guarantee that when you inform your son of that decision, he will instantly admit to being the culprit, thus tying up all the loose ends.

Visit family psychologist John Rosemond’s website at www.johnrosemond.com; readers may send him email at questions@rosemond.com; due to the volume of mail, not every question will be answered.