WHY SO many nuclear families explode is not anything Miss Manners cares to study. There are too many people in the field already, and Miss Manners has noticed that anyone who stands too close is apt to find something flying into his or her eye.

Miss Manners prefers to wait until the dust settles and then to study the new elements that have been formed, and their relationships.

It isn't easy. Miss Manners has been asked the following questions in recent weeks:

My son's ex-wife has remarried, and of course she is no longer my daughter-in-law. But I often see her because she has custody of my grandchildren. How do I introduce her to my friends?

I grew up with the daughters of my mother's second husband. My mother and their father are no longer married, but we are still on affectionate terms. They introduce me as their "brother," and I have been introducing them as my "ex-step-sisters." Are they exaggerating, or am I being too precise and cold? We all have a half-sister in common, and we all call her our "sister."

I often have dealings with my wife's ex-husband because he is the father of the children I live with. How do I refer to him when we make travel arrangements and so on? We get along fine, and I want a pleasant term.

All these people are trying to avoid the harshness of saying "ex." "Ex" is so final, and these relationships are obviously continuing, or there wouldn't be any problem.

The other prefix they don't like is "step." This was given a bad name by the publicity attached to the unfortunate homelife of Cinderalla, although dear Edith Wharton refers charmingly to "the steps" resulting from various marriages of her characters in "The Children," that marvelous jet set novel written before such people had ever set foot in a jet.

Other terms could be invented. One could refer to "the children's biodaddy," as a gentleman of Miss Manners' acquaintance suggested, or to "my sisters once removed." Another gentleman of Miss Manners' acquaintance calls his younger step-brother and sisters collectively, "My father's second litter."

Miss Manners, however, believes in recycling old words and rescuing them, if possible, from undeserved bad associations. Many kind people who devote their lives to children they didn't foist upon the world resent the name of stepmother or stepfather, which suggests that they hand out poisoned apples.

In some cases, the relationship can simply be briefly described: "The mother of my grandchildren," "the father of my stepchildren." In others, it is not necessary to define the relationship exactly and, unless those so designated object, there is no reason why the terms "brother" and "sister" cannot be used for the variations on this relationship. Provided the whole thing doesn't get out of hand and we all wander around sounding like a vast revival meeting.

But wouldn't it be rather sweet to hear one grown man refer to another as "the father of my children?"

MISS MANNERS RESPONDS

Q: The owner of the hair salon where I go regularly cuts my hair, and I do know that you don't tip the owner of the shop. I do know that you do tip the shampoo girl. But what about the girl who blow-dries my hair with that little handblower? When she's finished, the owner comes by and puts the final touches on my hair. It's no good watching the other ladies to see if they tip the blow dry girl because they have their hair dried under the regular machines. How many outstretched palms do I have to fill?

A: Miss Manners has never had her hair "done," or even cut, for that matter, and if you think tipping is difficult and expensive, try buying tortoise hairpins. She does, however, find it silly, if not degrading, to dole out lots of little bits of money to several people. You haven't even mentioned a colorist or a manucurist. Miss Manners suggests you add 20 percent to your bill and request that it be divided in appropriate proportions.

Q: How should a nonsmoker indicate his displeasure with those smoking in a public facility? I, of course, avoid such facilities when possible, preferring, for example, transport by private auto or airplane. Yet there are those times, banks, stock companies, and doctors' waiting rooms, when one must deal at the common level. Does one address the smoker directly or does one address that person in charge of the office so that they may convey the request? Should it be phrased as a request, a mild complaint, or a demand?

A: All of the above. The request is addressed to the smoker. The mild complaint is addresssed to the person in charge. And the demand is addressed to the assembly at large, or to God.