Do they have small kids? When my kids were little I temporarily forgot how to talk to adults. I'd catch myself starting to explain retarded stuff to them, like how to dial information. 
The worst offender, no, the second-worst offender does have a toddler.
The guy from the dentist conversation I referred to, I used to take offense to, and then he'd get defensive, and we'd end up in a big argument. Now I just explain patiently why I am taking the course of action I am taking and thank him, which is probably reinforcing his moronic behavior but it's much less of a hassle.
Sometimes it's frustrating to realize that I am explaining my mundane day-to-day choices to someone whose business it is none of, like yesterday, explaining to this guy why I was planning to write myself a cash-advance check on my credit card and pay the dentist with cash so I could get a discount, and then he says
Why not write it to the dentist?
and I am already irritated because he's being condescending but I say that it's because they do not offer a discount for checks for new customers and then he says
you ought to call and ask of checks count the same - they generally do, as they don't cost the recipient anything to deposit. in common usage "cash discount" means "discount for paying immediately and not using a credit card." I have received cash discounts at two different dentists paying by check.
NO SHIT SHERLOCK, REALLY? IT'S NOT LIKE I EVER OWNED A BUSINESS OR NUTHIN
and then I find myself explaining that the reason I didn't want to write the check directly to the dentist is because they don't give the discount with checks if you are a new patient because they have had problems with new clients bouncing checks for large dollar amounts and the collections expense offsets any savings from not having to run a credit card and halfway through explaining it I'm thinking WHY THE FUCK AM I EXPLAINING ALL THIS TO BOZO HERE?
