The other day, I asked one of my sons to put stamps on some utility bills while I went back to my bedroom to get my jacket.
"Daddy, tell me again why you put stickers on your letters," he said.
"Does he really not understand the concept of postage?" I thought. "Does he think letters arrive via the Tooth Fairy?"
Then it hit me. You can live a perfectly normal life nowadays without ever mailing anything. Even letters to Santa at our house are dispatched to the North Pole by e-mail.
Still, there's no denying that kids today draw on significantly different life experiences than I -- and perhaps you -- had as a child way back in the 20th century.
Come to think of it, here's how upside-down the world has gotten: In my corner of suburbia, Signal Mountain, it's not uncommon for parents to have the "sex talk" with a kid years before they have the "Santa talk."
I'll pause here while you let that sink in.
This, of course, raises the question that has baffled great thinkers for generations: How you gonna keep that elf on the shelf once he knows about the birds and the bees?
Sometimes it slays me what kids today know and don't know. In some ways they are smarter than their parents. For example, my older son, who's 12, knows more about ancient history and modern science than I do. My 6-year-old son, meanwhile, is self-educated in the culinary arts, having taught himself to be a competent baker by watching hours and hours of YouTube videos.
Still, it's jarring sometimes to think of all the cultural stuff, even the sensory detail, they missed by being born in the 21st century -- things that are embedded in my brain, like the soft whir of a cassette tape rewinding or the sticky sweet flavor of postage-stamp glue.
I set out to make a list of things my kids have never done.
As far as I know, they've never turned a channel knob on a television, gone to a public library to study or purchased a CD (much less a cassette tape or a vinyl record). They've never poured milk from a paper carton, slept outdoors or ridden a bike without a helmet.
They've also never used a stick as a play gun, ridden in a car without air conditioning or sipped a Coke from a glass bottle. They don't know how to make a kazoo with a blade of grass between their thumbs, nor have they ever walked to a neighborhood store alone.
On the other hand, there are things about their upbringing that I never experienced as a child.
I certainly never applied for a passport, spent part of my allowance for the chance to shoot other people with an air rifle or had a paid athletics coach.
I also didn't start thinking about the SAT in sixth grade or attend an elementary school dance.
Speaking of school, we didn't have zero-tolerance policies like they do today because we had zero-tolerance daddies at home. And don't get me started on money. Our cars today cost more than the houses we grew up in.
Still a few things have stayed the same, I suppose. Off the top of my head, I can think of one word and one style.
"Cool," used as an informal adjective, has held up for more than 50 years, making it the most enduring idiom I can think of.
And, boys' hair -- a sort of modified moptop -- is virtually identical today to when I was in junior high and high school in the 1970s. Thus I can pat one of my boys on the head after a haircut and say "cool hair." For an instant, we are communicating on the same wavelength.
At least it's a start.
Contact Mark Kennedy at firstname.lastname@example.org or 423-757-6645. Follow him on Twitter @TFPCOLUMNIST. Subscribe to his Facebook updates at www.facebook.com/mkennedycolumnist.