A couple of months ago, the sex education notice came home in my nine-year-old son's backpack. I didn't realize that, in our district, sex ed starts in the fourth grade. Another sign of the state having more access to my baby than I sometimes wish.

When I handed the note to my mate at the dinner table, our son said with something of a proud smile, "I told Mrs. Reverby we've already talked about it at home."

The mate and I looked at each other and obviously had the same thought. Two weeks before, the class had been learning about electricity. The teacher had gotten stuck on some questions about batteries, so she had turned to our son, who was able to explain to the class exactly how batteries charge, recharge, and discharge. He's learned a lot about electricity at home.

And quite a lot about sex.

"You know," my mate said to our son, "this is one of those times when you have to not help the teacher even if you know how something works."

I busted out laughing at the admonition. "Your dad is right," I said, composing myself. "It's entirely possibly you know more about sex than they do, but there's some stuff some parents might not want their kids to know, so you have to keep a lid on it."

The mate and I agreed the reference to HIV/AIDS must be code to tell us they'd be talking about homosexuality. What a way to code for our gay friends.

"I know," he answered.

But really. This was the kid who in preschool answered a teacher's "Good morning, how are you today?" with "I'm fine, but my mother is menstruating, so her uterine lining is sloughing." I just shrugged and explained to her that he'd seen blood on the toilet paper and wanted to know if I was OK.

So I had explained that it was normal, and he wanted to hear about the mechanics, like he always did about everything.

She laughed. As he went off to play, she reminded me of the time that the class had somehow gotten onto the discussion of baby cows, and one child had posed the question of how the cow gets out of the mommy's tummy. The teachers glanced nervously at each other until one of them sputtered, "Through the birth canal!"

My son's hand shot up: "Is that the same as the vagina?" Apparently he also pointed out that the baby must be in a uterus, not a tummy, because if the baby was in the stomach it would get digested, and that wouldn't be good.

This was also the only kid in preschool who said, "Most boys have penises and scrotums and most girls have clitorises and vaginas." I presume it is because my son knows so much about sex that sometimes his friends have tried to ask me questions. I never know what to do in such a situation.

Ordinarily I answer all children's questions in an honest manner and make sure I evince no shame about the question or the answer, whether it is about war, disability, disease, sex, arguments between neighbors, whatever. But in this cultural climate of negativity around sex, can I really answer another person's child's question about sex?

One day nine-year-old Elaine started asking me about birth control out of the blue. I said to her, "Listen, I need to call your parents and ask them if it's OK for me to talk to you about this, OK?" She said that'd be fine. So I did. I didn't expect her mother's response.

"Oh, God, yes, please answer any questions she has! And tell her it's OK to go to you any time with those questions!" I told her that'd be fine, but that I'd also ask Elaine if it was OK for me to just let her mother know what we had talked about.

My mate has always been a little more reserved with "adult" information. This is a general difference between us, one that's pretty apparent to everyone; a friend once asked our son what it's like to be raised by Auntie Mame and Kermit the Frog. But I have to be forthcoming with the goods, especially when it comes to sex. My work on children born with atypical sex has put me in the position of advising other parents that it is critical to be calm and honest in response to children's questions about sex. I kind of have to practice what I preach.

It's a problem, though, that I've become so comfortable talking with children about sex, because most adults aren't, and we've got a pedophile-panicked culture that just seems to be adding to the great silence. One time my son was out to lunch with a friend and me, and the friend and I were talking about my work on intersex. My son stopped me to ask me to remind him what intersex is. I explained we were talking about people who have a different kind of sex anatomy than the average boy or the average girl. I explained that, for example, some of them have a short penis or a big clitoris. "Oh, right!" he answered. I reminded him of the names of a few friends of ours who are intersex, so he'd remember we were talking about real people.

Suddenly I became aware that the tables all around us had gone silent.

Then there was the time in third grade when my son wanted to bring our pet rat Treacle in for show-and-tell. After my son and I had explained Treacle's care and feeding, his habits and his relations with us, one little boy had a question.

"What's that under Treacle's tail?"

"You mean those lumps?" I asked. He nodded. "Those are Treacle's testicles," I answered, not even thinking twice.

Pandemonium broke out. My son and I were baffled. "What did you say that was funny?" he asked me.

"I don't know," I answered, genuinely confused, as the teacher tried to restore order.

The boy who had asked the question piped up. "But I don't know what testicles are."

"It's where it really hurts when they punch you!" another boy answered, making a serious punching gesture.

"Great," I thought to myself. "Welcome to your genitals. It's where you get punched."

I asked the teacher later what she would have said in response to the question. "I don't know," she said, embarrassed. "I think I would have ignored it and moved on." I was stunned. This was a teacher I loved. This was a woman who, when one of her friends was dying of cancer, had been honest with our children about why she was so sad. She told the kids each day how her friend was doing, how much she hated cancer, and when her friend died, all the kids understood that she had to go to the funeral. She had taught our children a shameless view of cancer and of death. But she couldn't answer a question about testicles?

Our son asked why they didn't tell him this stuff at school. The mate explained that adults stupidly think that if you tell children the truth about sex, they'll have sex earlier than they really should.

She explained to me that she'd have to send a note home. In it, she mentioned what happened, and said that "Alice, in her usual forthright and honest manner, answered the question." And yet the note had a real tone of shame to it.

The note that came home about sex ed seemed to have a tone of shame to it, too: "According to state law, you have the right to review the materials and curriculum content to be used in HIV/AIDS and other serious communicable disease prevention education, as well as sex education." The mate and I agreed the reference to HIV/AIDS must be code to tell us they'd be talking about homosexuality. What a way to code for our gay friends.

I found myself hoping the gym teacher wasn't going to teach in code. Children spent so much of their energy learning not just the native language of their parents, but their coded language, too. I remember when the movie Juno was out, and a sudden rash of curiosity broke out among my son's class about what "accidentally pregnant" meant.

I realized why my son was confused. He was thinking "accidentally getting pregnant" was like accidentally burning yourself because you didn't realize the stove was on. "Sweetie," I explained, "most of the time that people have sex, they're not having it to have a baby. They're having it because it feels good. So you can get accidentally pregnant if you're having sex for pleasure and you don't use effective birth control."

He looked shocked. Apparently I had forgotten to mention that sex was not just for making babies.

"Think about evolution," I added (because he has also been raised a child of Darwin). "If the only motivation for sex was having a baby, we wouldn't have very much sex, and our genes wouldn't be passed on very much. But if sex feels good to people or to other animals...."

"Then they'll have a lot of sex and the genes will get passed down!" he said, finishing the puzzle. I nodded. He went on, "Do you and dad ever do it for that reason?"

"Most of the time we've done it or do it, it's for pleasure, honey." He looked a combination of fascinated and chagrined. "You know you were no accident. Before that, I went off birth control to get pregnant, and we were so happy when you came into our lives." He smiled because he could see me tearing up at how much I love him. (Aunty Mame cries a lot of love.)

He hemmed and hawed next, and I got the sense he wanted to know what I meant when I said it feels good. I asked him if he had that question. He said yes. So I said it was kind of like having someone scratch your back in a place that itched, and having them scratch it just right. I said that, after puberty, he'd know what the special itch felt like. He nodded.

So the morning of sex ed, I found myself wondering whether they were going to mention pleasure. Or would it be all about disease and pregnancy, all gloom and doom?

As it turned out, I'm not even sure they mentioned sex at all. Over bagels the Saturday morning following sex ed day, I started my inquiry by asking our son what he learned about HIV. "It's an inherited disease," he told me. "You get it from your mother."

The mate and I sighed. We explained to him that most people get HIV from sex or from dirty needles, and we explained about condoms, and about drug addiction, and about what makes a disease inherited versus contagious. The table of four adults next do us did that thing again; they went silent. We kept talking. Our son asked why they didn't tell him this stuff at school. The mate explained that adults stupidly think that if you tell children the truth about sex, they'll have sex earlier than they really should. He added that the evidence indicates otherwise.

And I was off thinking: How funny that we can't bring ourselves to tell our children the most fundamental truth about sex, that most of the time we have sex, we have it for pleasure. As I watched my son chomp on his peanut butter bagel, I was struck by the thought that I sure have learned from him how a single act of sex can give you pleasure for years to come. I can't believe he's supposed to give me a present for Mother's Day. Many days, I can't even fathom that he came from sex. He just seems magical.