On Friday, a dog bit a man.

This is an event so commonplace that it is the very symbol of a non-news story: Dog Bites Man.

We all know that dogs bite. They bite mail carriers, package deliverers, gas meter readers and even, from time to time, an electrician or two. It's one of the things they do, like digging for bones or licking.

They bite, therefore they are. Dogs.

Why was this event different from all other such events? Why was this so startling? Because it was our dog who bit a man.

Did I take it in stride? Of course. I reacted as calmly as if the police had called to tell me they'd picked up my daughter coming out of a bank with a machine gun.

Like a person who discovers that the downstairs neighbor is a mass murderer, I gasped: "Him? But he was such a quiet boy."

In short, I behaved with the same level of surprise shared by a million other pet owners every year when, in fact, their dog bites man.

Despite the absolute banality of it, there is barely a dog owner alive who isn't 1) shocked, 2) defensive, 3) protective when the faint odor of people is found on his or her dog's breath. Dogs, you see, belong to that peculiar category of modern beasts known as "pets." Their primary function, their reason for being, is to give and return affection.

Being fairly docile creatures, dogs learn quickly the same kind of behavior that fills the bill or, rather, the bowl. They do not bite the hand that feeds them.

So, over time, they train us to utter such absolutely inane things as "You know, sometimes I think he's human."

I have personally been heard to say "excuse me" when I pass our dog in the hall. I have also been heard to say at least two of the following statements: h

"Don't worry, he's just being friendly."

"If a thief came in, he'd probably want to play ball."

"He'd never bite anyone!"

The false image I lived with until lo this very Friday was imprinted in my brain in the darkness of the Saturday afternoon movies. In the Hollywood of my youth, all dogs were divided into two categories. There were 1) bad dogs who bit good guys and 2) good dogs who bit bad guys.

There were Doberman pinschers genetically programmed to salivate whenever a human appeared. I suspect to this day that they regard the human arm as just another salami.

There were more discriminating German shepherds in war movies. They bared their teeth and strained on their chains to "The Sound of Music." There were Lassie and assorted Disney dogs that only chomped down on certified kidnappers.

The dogs that we choose to live with -- unless we own a gas station -- clearly belong to the Disney world.

Our dog, for example (the one with a piece of blue jean between his teeth), is "good with children."

Our dog (the one that cost me $82 in doctor's bills) has his hair clipped at $20 a shot four times a year.

Our dog (the one currently nicknamed Munch) has a pedigree so long that he can be considered slumming in our middle-class suburb.

Need I tell you that this incident, this sudden revelation that the dog who eats mail might also eat a mailman, dealt a blow to our pride and our favorite myths?

We ask ourselves: Where did we go wrong? Was he making some desperate ploy for attention? Was there some social message that he was communicating with his teeth?

But no. Finally, in the dark night of our soul-searching, we realized that we had forgotten only one thing: Dog Bites Man.

Now we have come to terms with reality, come to accept the fact that our Zachary is, after all, an animal.

It is only when I look deeply into his brown eyes that I find myself wondering: What did the electrician do to make my precious poochie so cross?