There comes a time in everyone's life when certain kinds of corporeal questions loom large. Young people may not go as far as puppies, sticking their noses in every available crotch, but the dirty parts of your library may start looking dog-eared. When children get to that giggly stage, many parents head for the bookstores, hoping to replace "Fear of Flying" and "Measure for Measure" with something that has more up-to-date information about birth control and AIDS.

To anyone over 19, kids' books about sex will seem uniformly boring. Except for the chapters on sexually transmitted diseases, most of the books are little different from the ones my friends and I did our health ed homework from 20 years ago in Ms. Condon's class. (Tired of stressing the n, I suppose, she went back to her maiden name the next year.) There are the same mysterious cross sections of the male and female pelvis, looking like a bus map of Rome, only harder to follow. There are the same cartoons of grinning sperm elbowing their way to the coy, well-coiffed egg. There are the same exhortations to bathe frequently, refuse spiked punch, refrain from popping those zits and learn to be a good listener. Even the cautionary tales of Tanya, who gave in when her boyfriend said "please" and is now faced with a Difficult Choice, or Josh, who didn't wear a rubber and now has a Terrible Disease, seem bleached of emotion, as if such things could never happen to a living teenager.

Do the authors hope that if they make their books dull enough, kids will be too bored to have sex? It didn't work then, and it won't work now. More likely, though, the dullness is meant to reassure. Red jelly may be pouring out between your legs, pimples may be sprouting from your forehead like crocuses, your voice may be channel-surfing up and down the octaves, but don't worry. It's all perfectly normal -- "It's Perfectly Normal" is even the title of one of the better books. (Of course, what with hormones and the novelty of the subject, children won't find these books half as dull as adults do.)

Perhaps the most important unspoken function of boring sex books may be to reassure not kids but parents. And after all, as some of the authors point out, children aren't the only nervous ones. In "What's the Big Secret?" a picture book for young children, a little boy asks the classic question. "Umm. Uh, well, you see ... I want to tell you, except ..." stutters dad. Excuses rim the margin ("You won't understand. It might frighten you. Mom will know what to say. My throat hurts. There must be a book about it"), while the center of the page tells the short version of the old story.

Although the books generally stress the importance of talking to your kids (or parents) about sex, grown-ups sometimes use them as a substitute for conversation, not just a supplement. So the books have a tough job. They have to appeal to parents whose sexual attitudes range from those of hippies to Shakers. They have to instruct kids about the life-threatening dangers of sex without giving them lifelong hang-ups. They have to include sections on how to say no without coming across as hopelessly prudish. They have to draw readers in without spoiling their credibility by obvious preaching or dated slang. No wonder they take refuge in lengthy discussions of the shape and function of the epididymis.

Enlightened, chatty, liberated, open-minded parents may be thinking: Forget it. Why should I waste my money? I'm not one of those irresponsible adults who expects a book to do my job for me. My children already know the facts of life; if they need any additional information, they can ask me.

But they may not want to. If adolescence is a time to slam doors, risk punishment by staying out past curfew and show off one's independence, it may not be such a great time to let one's parents see one's ignorance of a subject that defines adulthood. And kids have a natural reluctance to think about their parents' sex lives. Any question you answer will probably conjure up images of you nude and passionate -- yuck. Furthermore, however studly you might be, your view of sexuality could be narrower than you think. For the sake of your children's privacy, of providing them with a variety of points of view and of giving them access to Latin names for body parts, it's a good idea to leave a few sex books in discreet, accessible places.

Make sure you read them yourself first. While these books are careful to debunk many harmful old myths about sex -- that you can't get pregnant if you have intercourse standing up, that masturbation will make you sterile -- most of them can't resist adding a few of their own. Some imply that sex is only pleasurable when it's between two people who know each other well and love each other; some say penis size makes no difference; some insist that even if you daydream about members of the same sex, you shouldn't worry, you're not gay. Parents and authors might have good reasons for wanting their children to believe these things, but that doesn't make them always true. It should be possible for authors -- and parents -- to say honestly, "Sex with strangers may be exciting, but don't do it: It's too dangerous," or, "Different people have different preferences."

"Changing Bodies, Changing Lives" is a comfortingly honest book. The authors, who also wrote "Our Bodies, Our Selves," talked to a raft of teens and include their voices in a nonjudgmental, respectful way. Although it's due for another update -- the most recent edition is from 1987 -- it's probably still the best of the bunch for older children and teens, assuming they can get past a fairly high dork factor (lots of braces, terrible poetry and people "digging" things). Each section has a bibliography -- kids may find it a revelation to learn that more information is available. However, since the information on AIDS is more than 10 years old (as are the hairdos), it's a good idea to supplement the book with more recent material.

Books for younger kids may mention problems such as AIDS, rape, incest and unwanted pregnancy, but they don't go into much detail. Ones for teens try to strike a balance between terrifying their readers with grisly details and leaving out things that readers may urgently need to know. With all their reassurance about how normal adolescence is, these books may not be as useful for kids with uncommon experiences or problems. That's one reason books with good bibliographies, like "Changing Bodies, Changing Lives," are so important. Many recommend places to call for help -- rape hotlines, Planned Parenthood -- and sometimes even include phone numbers.

For less life-threatening questions (Why is there hair on my ears? Where can I go that's really private? What's that funny smell? What do I do when men make rude comments about my body? How come my menstrual blood is sometimes red and sometimes brown?), books with Q&As can give readers a sense that they're not alone. "Girltalk," by Girls' Life columnist Carol Weston, has easy-to-identify-with "Dear Carol" letters and passages from the author's high school diary. "I came close to having sex," writes one girl. "We were in a camper. He tried to get it in, but it wouldn't go in. Is that normal, and am I still a virgin?" Carol answers: "A girl is a virgin until she has penis-inside-vagina intercourse. It is normal for penetration to be difficult in the beginning. Some girls have trouble because their hymen is still intact. Others have trouble because the couple is in a hurry and the girl's body does not have time to respond sexually. Lubricated condoms can help. Better still, put on the brakes until you are really ready, physically and emotionally." "Can a gynecologist tell if one has masturbated?" asks another. "Nope."

"What Kids Really Want to Know About Sex," from Britain, answers many touching letters about love, body hair, ill-timed erections, masturbation fears, strict parents and other woes. For example: "When do you get milk in your breasts? We are three worried 11-year-old girls who are scared that when we have a baby, we might not have any milk to feed them." Or: "I'm a boy of 13 and I'm particularly worried about a part of my body. On my penis I have two veins which start at the top of my penis and finish at the beginning of my testicles. These veins stick out a lot. Am I abnormal and can I do anything about it?" The author answers the questions seriously and straightforwardly, taking care not to embarrass the questioner: "These veins have to work very hard and need to be thick to carry the blood swiftly round this part of your anatomy. All men have them." American readers, who may have little firsthand knowledge of the subject, might find the sections about uncircumcised penises intriguing.

The Q&A books give children a sense of the variety of concerns other kids have about sex, hopefully making them less embarrassed about their own questions. Particularly for younger kids, books with lots of pictures are important to give a sense of the variety of bodies out there. Unfortunately, illustrations tend to be scanty; perhaps authors and publishers are afraid of making their sex books seem too sexy. "It's Perfectly Normal" is an exception. Not only do the illustrations throughout the book show attractive, unidealized folks of various shapes, but it even has a spread of cheerful nudes in all colors, some with muscular thighs, some with pot bellies, some with walleyed breasts, some with crutches.

Want more sex advice? Here are some things I wish Ms. Condon had told me (though would I have listened?):

Wash out blood with cold water; hot water sets the stain. Don't assume everyone else is having a happy, comfortable, hot sex life: They may be, but they also may not, and you won't know that unless they tell you. Just because you like imagining a particular sex act doesn't necessarily mean you like to actually do it. Many people have sex with only one or two people in their entire lives; others have dozens or even hundreds of partners. If it hurts, stop. Just because you think something is gross now doesn't necessarily mean you always will. No one can know what you're thinking unless you tell them, even if you had a wet dream about them or are imagining what they look like without their clothes. Teachers who sleep with their students are pathetic losers. You may have to use your hands to guide the penis into the vagina. If you're inexperienced and embarrass easily, the best partner might be someone who makes you laugh. Brush your tongue as well as your teeth. If you're really worried, see a doctor; if that doctor doesn't help, see another one. Crying can be fun. Wait until you feel comfortable -- believe it or not, you'll always have another chance later. When someone turns you down, that doesn't mean there's something wrong with you. Not everyone likes having their ears licked. The pimple on your chin looks much bigger and redder to you than it does to anyone else. Wait until after your best friend ends that relationship before you have sex with their partner. No matter what you look like, somebody out there is especially attracted to people who look like you. You're still the same person you were yesterday. If you don't ask, you won't find out. If your partner keeps treating you badly, break up. You'll look better if you stand up straight. Your parents are not always right. Your parents are sometimes right. Whatever your problem is, there's someone else in the world who has it too.

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B O O K +I N F O R M A T I O N:

What's the Big Secret? Talking about Sex with Girls and Boys
By Laurie Krasny Brown and Marc Brown
Little, Brown