Julia Sweeney considered herself an enlightened, sex-is-no-big-deal kind of parent. But that was before an innocent question about tadpoles prompted The Conversation

But what do they do with their legs?

One evening, on a school night, when my daughter Mulan was nine, we were eating dinner together at our favourite Thai restaurant. It was autumn, over two years ago, and writing about it now I see that Mulan and I interacted much like two roommates. We ate out a lot. We had a handful of favourite places. When you're a single mother who primarily takes her daughter to dinner at restaurants (my meagre defence: I was spending four days a week driving her to gymnastics after school – 45 minutes each way – so, who had time to cook?), it's easy to think of yourselves as a couple. You eat, you talk, and sometimes you just stare at each other in a stupor of familiarity.

At the restaurant, we know the owner and chef, who this night recommended the frogs' legs in hot peppers. We politely declined. Mulan told me her class had begun studying frogs. In fact, she revealed she had a report to do, and began to explain the basic parameters: "So, Mum. First, the frogs lay eggs, in a pond, and then the eggs turn into tadpoles and the tadpoles turn into more frogs."

I squinted my eyes. Biology – and science in general – was not my academic strong suit. Only recently had I discovered my own deep, neglected interest in science, and had been scrambling to catch up with the 21st century. Whenever Mulan told me of anything she was learning about science, I'm sure I wore an expression of astonished bewilderment and surprise. My 12 years of Catholic schooling did not dwell long on biology (God didn't want us thinking about that) and avoided the subject of reproduction almost entirely.

Eventually I mumbled a response: "Uh… yeah. I think so. I think, though, that it's probably just the females that lay the eggs, and then the males fertilise them – although I don't know for sure – and there are probably all kinds of species of frogs with different ways of doing things. But yeah, in general, I'm willing to bet, the females are the ones with the eggs. Or something like that."

"Huh?" Mulan said, listening carefully. "But, what does 'fertilise' mean?"

I said, "Oh, the males have this substance inside them, and it's like a co-ingredient, called sperm. They sprinkle, or squirt it on the eggs. That's how they get fertilised. It takes both the female's eggs and the male's sperm, and together they make the new tadpoles." I was really proud of myself for the word "co-ingredient". That was good.

"Soooooo, only the females have the eggs." Mulan said, her eyes wandering to the ceiling, taking this all in.

"Yes," I said.

"Humans, too?" she asked.

Let me freeze this scene for a moment and say that I considered myself an enlightened, open-minded, sex-is–no-big-deal parent, yet I hadn't truly prepared myself for this conversation. I had read a few parenting books and they all seemed to advise the same thing, which was, when your child starts to ask you about sex, or really anything that is complicated and multifaceted, just answer the exact question they ask. Nothing more. Don't elaborate. Don't over-share.

In that sense, I suppose I was prepared for this crucial rite-of-passage. I wasn't going to stop and take her hand, get all watery-eyed and explain about the beautiful way we create more children in the world. That wasn't what she was asking. She just wanted to know if human women had the eggs. The answer was clear and unambiguous.

"Yes," I said. I deliberately forced a pause. I tried to think of some other subject to move on to. I took a big bite of the mango salad we'd just been served.

Mulan asked, "Where do women keep their eggs?"

"Well," I said, "we women have evolved to have our own pond, right inside our own bodies. We lay our eggs in this pond, which is so convenient when you think about it compared with frogs, because we don't have to worry about any competing eggs. It's a pond of our own."

A pond of one's own. I imagined Virginia Woolf contentedly sitting in a pond of her own. And then drowning.

"Where is it?" Mulan asked, her eyes bigger than ever.

"It's in our lower abdomen, inside us, below our belly button, above our vagina." I had managed to be specific and totally vague all at once. Perfect.

"But… how do the eggs get fertilised?"

"By the man," I said, thinking why did I use the phrase "the man"? Aside from its conformist big-business connotations, I had possibly implied that there was only one man, some special Man who was used only for this purpose. Creepy and weird. And, of course, incorrect.

Thankfully, at this moment the rest of the food was delivered. I scooped up some green beans with chilli and hoped the subject would change. I realised my eyes were darting around, which reminded me of my own mother. I hated how awkward and embarrassed and offputting my mother became about the subject of sex. Now my own body was displaying the same indications of unease. I took a deep breath and smiled in a deliberately relaxed way at Mulan.

"But how does the sperm get in to fertilise the eggs?" she asked.

I said, "Oh, yes. That. Well, the sperm comes out of the man's penis and it goes into the woman's vagina. This happens when the two do what's called, 'have sex'. And that's where the egg – there's usually only one in the woman's pond at a time – gets fertilised." Only after the fact did I realise that I had said the words penis and vagina and sex in a strained, sotto voce tone. This was also something my own mother would have done. Self-hate swelled in my breast.

Mulan had put down her fork. Her face was twisted in disgust. "That's where humans make a baby, where you go to the bathroom? Mum!!" Her voice was rising.

"Yes," I said, looking around conspiratorially. "I know," I sighed. "It is weird. That part can take some getting used to."

"Gross." Mulan mumbled.

"Yeah, I know. As they say, it's like having a waste treatment plant right next to an amusement park. Terrible town planning."

"What?" Mulan said.

"The thing is," I went on, "that's how we evolved. That's where it all happens. And even though going to the bathroom and having sex are both in the general same area, they are actually totally separate." I wanted to add, "Except for some people, where psychologically it gets all mushed together, which is creepy in my opinion but certainly not morally wrong, and is actually understandable given the proximity." But that seemed to be getting ahead of the conversation, so I tried to change the direction slightly.

"Like your nose and your mouth," I ventured. "They're both close to each other on your face, but you wouldn't stick a bean sprout up your nose." Mulan gave me a pathetic lower-teeth-revealing smile and grunted a charity chuckle. Then she got back to the topic at hand.

"But Mum," Mulan asked with tractor-beam focus, "how can this ever happen? I mean, men and women, they can never be naked together."

"Well," I explained, "when people are older – much, much older than a kid – when they are older and they both decide they want to, in very certain circumstances, like if they're in love with each other, well, then, they can be naked together."

"But how do they know when?" Mulan asked. "Does the man say, 'Is now the time to take off my pants?'"

We held each other's gaze for a moment.

"Yes," I said. "That's exactly what they say."

To my great relief Mulan seemed content with that knowledge and began to eat with gusto. We moved on to other topics of conversation.

As we drove home Mulan seemed unusually quiet. I glanced at her from time to time in my rear-view mirror. She was sitting in the back seat, staring out of the window. The pavements were filled with people.

Suddenly Mulan laughed.

"What?" I asked.

"Oh Mum, you're going to laugh so hard."

"Why?"

"Because, Mum, you can't believe what I thought you said back at the restaurant. It's so funny. I thought you said that the man puts his penis in a woman's vagina – inside of it – and that's how people make a baby. Isn't that hysterical?"

A pause.

"That is what I said," I said.

"Oh," Mulan said. Her face had turned from gaiety to seriousness. There was a long quiet time. She stared out of the window, taking all this in.

Mulan asked, "What if two people just walked up to each other on the street and started doing it?" Our eyes met in the mirror. Her eyebrows were furrowed and she broke our gaze and looked at some people standing on the street.

At this point, I decided the best way to approach these questions was to pretend I was some dispassionate anthropologist discussing the mating habits of an animal other than our own. "The human species is very private when it comes to sex. Humans are unusual in this way. They have sex in private."

Mulan asked, "What if you went to a party and there were a bunch of men and women and they all just started doing it? Would that ever happen?"

"No," I lied. "That would never happen. Because humans are so private."

My back stiffened. I realised it stiffened like my grandmother's, my mother's mother. I was reaching back, farther back than my own mother's discomfort and into the graves of the next generation of discomfort. The dead live.

"Mum," Mulan said gravely, "have you ever done this?"

"Yes," I said, flatly.

"But Mum, you can't have children."

"That's true," I said.

"Well, you never have to do that again," Mulan sighed. She sounded relieved.

After a moment I said, "Well, if you really love someone and you're an adult, then you want to do it, even if you can't have a baby."

Silence. Mulan stared out of the window deep in thought. "But Mum, how can people do that? I mean, how do their legs go? You know, not everyone can do the splits."

Ah, the perspective of the proud gymnast. Mulan became somewhat fixated on the role of legs in sex. She could not picture how it was physically possible, even if someone could do the splits. Finally, I said, "Mulan, people figure the legs out. They just do."

"Oh," Mulan said, taking this in. She quieted down and we got home. When we got out of the car, our cat Val was sitting in the front garden soaking up the last bits of sunlight. Val rolled on to her back.

"What about cats? How do they do it?"

"It's basically the same idea," I said.

"But how do their legs go?" Mulan wondered.

"They, well, I think the male stands behind the female and… and… they just do, Mulan," I said, exasperated, and disappointed that "They just do" was the best I could do.

Once inside the house, our dog Arden, delirious with glee at our return, jumped up and licked my hand. "What about dogs?" Mulan asked, having never considered the possibility before.

"Same thing," I said. "It's basically the same thing for all mammals."

"But what about their legs?" she asked again.

"Look," I said, now desperately tired of this subject, "I've lost my ability to describe it. Maybe we can look on Wikipedia or something and it will show us."

So, we went to my office and got online. I Googled "cats mating". And, of course, on YouTube there were thousands of videos. We watched a couple of them. Mulan was riveted. She moved her face closer and closer to the monitor.

"Now what about dogs?" she asked. We watched a few dog videos. She put her hand on my arm.

Here, dear reader, we come to another moment out of time. Such as when you're in an accident and time slows to a crawl. I could hear my own breathing as if I were suddenly wearing a space suit from 2001: A Space Odyssey. Mulan's hand seemed to be reaching out to my arm in slow motion: frame-by-frame. I believe I remember it this way because it wasn't until then, until this small intimate gesture, this gesture of familiarity and of safety, that I realised where I'd led us.

"Mum, do you think there would be any videos of humans mating on the internet?"

I am a monster. An incompetent monster of a mother.

I smiled and said, firmly, "No. There would never be anything like that. Because humans are so private." And then, "Hey, how about some ice-cream?"

Which, of course, was teaching her that when questions about sex got awkward, food was truly the answer.

Later that night, Mulan asked, "What about Roger and Don – how do they do it?"

"I… I don't know," I said.

All right, I was thrown. I thought I would have more time between frogs and same-sex intercourse than just an hour or two. I was out of my depth.

Mulan went to the bathroom and took a little longer than usual to come out. Later she said, casually, "I think I know how Roger and Don do it."

"Oh yeah?" I said.

"Yeah, Mum, there's another hole down there, where you also go to the bathroom. Maybe… you know, maybe they use that."

That's my girl, my Mulan, age nine, inventing anal sex. Smart, inquisitive, problem-solving, Spock-like in objectivity and with a total lack of squeamishness. Bless her heart.

"Maybe," I answered, and shrugged my shoulders to indicate: see how casual and easygoing I am?

"But Mum," she said, "what about two girls? What about Eileen and Karen, how do they do it?"

"I... I…" I answered meekly, beaten.

"Why don't you call Karen and ask her?" Mulan asked me.

"Nah," I said, pretending to read the newspaper.

Mulan put her face a few inches from mine. She looked disgusted with me. "Mum, aren't you even curious?"