Planet Teenager, where I unfortunately live, is a busy and surprising place.

On this planet, kids come and go at all hours, but nobody ever rings a bell or knocks on a door. Nobody calls to warn, "I'm on my way." Nobody beeps a horn to say, "Your ride is here."

Instead, teens suddenly appear in my living room (often around dinnertime), only to vanish again without so much as a "Sorry for barging in" or "So long, it's been good to know ya!"

How do they manage to infiltrate my house without my knowing it?

Simple: They text on arrival — TOA — and another teenager lets them in.

To tell you the truth, it used to freak me out. Suddenly my 17-year-old would bolt from the table and open the door without a word, and to my utter astonishment, another teenager would be standing on the threshold.

"But ... but ... how did you know he was here?" I was foolish enough to sputter the first time I witnessed this some months ago.

The answer, of course, was that texts had been silently exchanged, rendering obsolete the ridiculously old-fashioned routine of ringing a bell, knocking, or even phoning to say that one's arrival was imminent.

Now my only hope for anticipating the appearance of guests is the dog. Dear old Buddy, whose extraordinary hearing and Doggie ESP can detect the lightest footstep, still twitches an ear and sometimes even pads over to the door if someone is about to enter. Since I am never the one receiving the "I'm here" texts (or more likely "Yo"), Buddy is my beacon, my four-legged heads-up.

I'm not saying TOA is a bad thing. I'm just saying, for a mom who lived most of her life in the 20th century, it takes a little getting used to.

As my niece who's in college pointed out to me, teenagers aren't the only ones who TOA. But based on my observations, the practice hasn't yet caught on among grown-ups — uh, I mean, old people. (Actually I mean middle-aged people, but I'm sure to a college student, middle-aged people are old people.)

Yes, I still ring the bell when I go to someone's house, fuddy-duddy that I am, and my friends (not that anyone under the legal drinking age believes I actually have friends) still ring the bell when they come to visit me.

But I have to admit, TOA has its advantages, especially for city-dwellers. How many times have I rung the bell to get into an apartment building, only to realize that the bell wasn't working. In the old days, you either had to slip into the building behind someone else who had a key, or find a pay phone on the corner to call your friend. Nowadays, of course, you could call your friend on your cell — or you could just TOA.

Another advantage to TOA: Nobody has to sit in the car in front of a house beeping the horn to show that Cinderella's coach has arrived. (Or if they did, you'd be well within your rights to open the window and scream: "Lay off the horn, will ya! Can't you just send a text that you're here?")

Still, the way teens appear and vanish thanks to TOA reminds me of the old TV show "Bewitched," where the witchy housewife Samantha's supernatural relatives were always conjuring themselves up in her living room, without using normal means of access like the door. Naturally this annoyed her mortal husband no end.

But now that TOA is the new normal, I predict the day will soon come when children watching a classic movie or old sitcom will be unable to understand the cliched line "Don't you ever knock?" — usually said in an angry voice when neighbors, roommates or nosy relatives barge in at awkward moments.

In fact, in sitcoms of the future, when someone barges in unannounced, instead of asking "Don't you ever knock," I wouldn't be surprised if the characters instead say "Don't you ever text?"

Beth J. Harpaz is the author of several books, including "13 Is the New 18."