Most parents make some stab at teaching their children to share their toys--a gesture that goes directly against human nature and is therefore an important step in the opposite direction, toward civilization.

But when it comes to sharing their own toys, many of the same parents simply give up, let the children take them over and then sulk about it. Miss Manners knows many households in which the telephones, television sets, phonographic equipment and hair dryers have simply been duplicated to avoid the problem of sharing between parent and child. Sometimes each child is issued this equipment.

This strikes Miss Manners as being rather an expensive way of dealing with the situation. And as computers increasingly become a part of household equipment, it is also ceasing to strike parents as the easiest solution to go out and purchase several.

So the parents will have to learn to share and even to teach sharing. This time around, it is harder because the concept of strict equality does not--should not--exist between parent and child. Is it fair, after all, that one of them should have to do all the child-rearing?

Let's see if we can do this without resorting to the ugliness of "Well, it belongs to me. If you want one, go out and buy your own."

The first question is whether the person who wants to use the equipment knows how to operate it properly and take care of it. Miss Manners has always thought it a shame that manufacturers of television sets do not make them impossible to use by anyone who cannot read and understand complicated instructions (the way the subway system is).

The next question is whether the use of the appliance will annoy others. That is usually in the nature of the item. Phonographs annoy everyone who did not select the record; dishwashers never do.

So far, the standards apply to everyone. Enough of that.

The question of whether the activity interferes with something else that the user ought to be doing applies only to children. Children with homework to do cannot be on the telephone (although they all claim they are only inquiring around to find out the homework assignment, which no one in the class seems to have heard). Parents with tax forms to fill out may chatter all they wish. Children who have to be up early to go to school may not watch television until all hours of the night. Parents who have to be up early to go to work may, because what they are actually doing is "unwinding."

Even less fair are restrictions on items that might logically be shared, but which the use of by others drives the owners crazy. Only grown-ups are allowed to have such quirks, and even then they are allowed only a few that they do not have to explain, with however twisted logic. Miss Manners does not like others to use her typewriter, however closely related they are to her or more skilled at operating it.

Children are allowed to apply this privilege only to appliances they have bought with their own money. A child who has bought a phonograph with summer job money is allowed to stipulate that no one else use it without his permission--a stance that would look dreadful in a parent.

Miss Manners does not pretend that this evens things out. But time will.

Q. Please comment on the "call waiting" service available for residential telephones. I find it very irritating to be interrupted by "call waiting" and to have the person I'm talking with switch to the waiting call.

A. It is perhaps part of the general optimism of the human race that most people cherish the notion that the next telephone call is bound to be more rewarding than the present one, or the next person at a cocktail party more interesting than the one to whom one is talking.

Or perhaps it is just part of the general rudeness of the same people, to whom dropping a live visitor or customer in favor of one who appears on the telephone has become commonplace.

In either case, Miss Manners shares your resentment. She would curse the whole abominable invention, if not for the certainty that people would deluge her with instances of emergencies in which the ability to break through to a busy line has saved orphaned widows from certain death.

The answer is that it should be used only for emergencies. If one asks one's caller to "hold" to take the other line, and the second caller does not say, "Your house is on fire!" then one must tell the second caller firmly that one will call him back later, and get back to the first call.

As the victim of this, you have Miss Manners' permission to hang up if the person to whom you were speaking does not get back to you in the time this would take. You would not be rude to do so--you would be recognizing that a state of emergency must exist, and would be freeing that line for him to call the fire department.

Q. Last week a friend and I went to a ladies' lunch and style show. We had no idea what the menu would be, but there were two teaspoons, a knife and two forks at each place. Not salad forks, but dinner forks.

Therefore, when the first course was a fruit cup--fresh fruit served in a very small glass dish on a saucer--we picked up one of the spoons and started eating. But looking around the table, we saw everyone else eating with one of the dinner forks. Checking the other tables, we saw it about evenly split between spoons and forks.

Which is correct, considering our choice of a dinner fork or a teaspoon, and that we did not know what the other courses would be?

A. You do not give Miss Manners a wide enough choice when you empower her with the privilege of bestowing blame. The party at fault here is the one who set the table and planned the menu. The rest of you may all be excused for making the best of a bad bargain.

Fruit cup is not, properly speaking, a first course. It is a dessert. This will come as a shock to all managers of hotel banquet rooms; as a matter of fact, if Miss Manners weren't such a stickler for form, she would admit to preferring cut-up raw fruit, to begin a meal in such a place, to lukewarm soup. Being a dessert, it is properly eaten with a dessert spoon and fork, not a teaspoon or a dinner fork.