WHY CHILDREN are such reluctant conversationalists is something Miss Manners Finds difficult to understand. They are handed so many good openings. What adult would be able to resist chatting merrily for hours after such sure-fire lead-ins as:

"You're really growing up, aren't you?"

"What are you learning in school these days?"

"Have you been good lately? Doing everything your mommy and daddy tell you to?"

"How do you like school?"

"How old are you?"

"Now, let's see -- do you look like your mother, or like your father?"

"How do you like having a new baby in the family?"

For some reason, most children respond to such promising overtures by looking at the floor and saying, "I dunno." If they had any sense, they would seize these opportunities to discuss the state of modern education, the future of the nuclear family, genetic engineering and the new morality.

But they don't, probably because they are children. And as they don't seem to be able to help that, perhaps it is up to the adults to assist them.

The easiest way to do this is to use on children the same conversation standards that you have for real people. You needn't discuss the stock market with toddlers, although they tend to gurgle when you do in Miss Manners' experience, probably because they have nothing to lose and can therefore afford to find the whole thing amusing.But special slants for children are invariably unproductive, and general opening remarks, which are rarely of complicated brilliance anyhow, tend to work better.

For example:

"What do you think of all this snow we've been having lately?

"Seen any good movies?"

"Inflation hitting you badly?"

Miss Manners dares to say that any one of these would serve to get some sort of conversation going with a child, especially if you explain that inflation means that your allowance isn't going as far as it used to. Then it's just a matter of remembering that ordinary rules of politeness and refraining from making the comments too personal and therefore patronizing. "I love your dress" is a polite remark; "My, what a big girl you're getting to be" is not. "What did you think of the show?" is a sensible question; "Didn't you just love the pretty snow?" is not.

However, until all grown-ups reach this standard, it will be necessary for parents to teach their children how to respond to the dumb questions.The most polite way, it seems to Miss Manners, is to teach them to make direct responses to the questions and then to turn the conversation away from themselves by making the sort of inquiries the other person enjoys.

In the case of most adults, the children reasonably expect them to be charmed by being asked such questions as:

"How old are you?"

"How do you like your job?"

"You're really getting bigger, aren't you?"

"Been good lately? Doing everything your spouse tells you to?"

"What have you learned at work lately?"

"How do you like having a new baby? And by the way, who in the world does it look like?" MISS MANNERS RESPONDS

Q.Is it proper to give an engagement ring if two widowed people in their early 60s have marriage plans?

A. Neither age nor previous degree of servitude should bar one from that peculiarly satisfying combination of romance, respectability and material gratification that is represented by an engagement ring. In a word: Yes.

Q. What do you do if you are visiting friends and your baby throws up on their rug?

A. Clean it up. What did you think -- that you should smile indulgently and finish your drink? Only the parents of newborn babies consider wetting and throwing up to be natural functions; your friends, no matter how many older children they have, find it uncommonly distasteful and probably ill-bred. If you cannot spare them the event, at least erase the consequences.

Q. My sister was reading a book that she kept reading parts of aloud to me. At first this annoyed me while I was trying to do other things, but then I really got interested. So one day when she was out, I started to read it. She got angry one day because I took it to the dentist's to read and didn't get home till after she was back and wanted it. OK, but then she said I can't read it at all until she's finished, which might be a long while because days go by that she doesn't have to read, and I could be. The book doesn't belong to either of us, it belongs to Mother, who said we should ask you to work out a schedule that is fair to both. Remember that I wouldn't have started it if she hadn't got me going.

A. Nothing could be fair. It is as impossible for two people to read the same copy of the same book as for two bodies to occupy the same space at the same time. Why don't you begin another book -- and tantalize her by laughing aloud at it?

Q. My girlfriend and I have been living together for a year and a half, which is a pretty permanent relationship by today's standards, and a lot more so than some marriages we know. Her parents were upset with us for a while but now they seem to be reconciled. At least they have invited us to stay with them. They live in a big old farmhouse and I have no objection to going, in spite of my distate for being "looked over" as, I'm sure they would like to think, a prospective son-in-law. My question is an elementary one. Do we have to sleep apart?

A. The hosts designate the sleeping accommodations for their guests. Unpack your suitcase in whatever room they say, whether or not it contains their daughter. That is properly their business. If you should get lost after bedtime in the hallway and stumble into wrong room, that is your business.

Q. Who opens the door for whom these days? The question may be deceptively simple.

As a matter of practicality, I tend to open a door myself when I am the first of a group to reach it, and then I keep it propped open for anyone who is following at a reasonably close interval. Recently, however, I have been called to task for such conduct by handicapped persons and sweet little old ladies who found it demeaning to have a door held for them. I certainly never meant to offend these fellow travelers.

On the other hand, I have nearly had my nose flattened when someone, regardless of my approach, has suddenly let a door swing back precipitously. Perhaps you could say a few words to those of us who stand perplexed at life's portals and who, for reasons of decorum (and working on an upper floor) have decided to eschew the window as a means of egress.

A. We are all passing down the hallway of life, some of us more quickly than others, and many innocent people are being trampled along the way.

We started, if Miss Manners may be allowed to drop a metaphor that wasn't going anywhere, with the fine principle that the strong should defer to the weak -- but by defining the "weak" as those who are female, disabled or aged. Perhaps we will arrive, one day, at judging the individual's weakness before applying the principle, so that a fully independent sweet little old lady with a cane will defer to a phlegmatic young man. It does seem to Miss Manners rather a nuisance to have to make such assessments at every doorway, but there is no denying that it would be more fair.

In the meantime, is to too much to ask everyone to be courteous to everyone else? There is not so much consideration in the world that anyone can afford to reject any bit that is offered. Please continue to hold doors for those behind you.

Q. Oh, boy, did I get it when I tried on my new formal for my friend and she asked me what shoes I was going to wear with it, and I said my good shoes, which are high heels in black leather with a bow. I admit that they are pretty worn and I am ready to buy a new pair. But that means I need new regular shoes, and what she is horrified at is the idea of my wearing "daytime" shoes with my evening gown. I can't afford to buy two pairs and still have the daytime ones be good ones that will last. (My friend's parents send her a check whenever she asks so she doesn't even think about the money). Is my original outfit so bad? Would you be horrified, too, if I wore the "daytime" shoes?

A. Yes, but Miss Manners has the knack of expressing horror and plotting economy at the same time. At the moment, suede shoes are fashionable for both daytime and evening. Seize the moment.