“What are those?” The younger asks, spying a packet of pads in the newly organized bathroom cabinet.
We’ve had a version of this conversation dozens of times. But she always acts as though she’s asking for the first time.
“They’re for when you have your period.”
“What’s a period?”
“It’s when you have bleeding, when you’re a wo—” I stop and revise mid-sentence, feeling pressure to be precise, “starting when you’re an older girl, once a month.”
I have a longer answer to this question, about the uterus and the lining blah blah blah, but my approach to explaining periods to the younger is similar to the way I approached telling people about my dissertation when I was applying for academic jobs. My answer exists in interlocking sections; I’ve got my one-sentence version, and I don’t offer elaboration until prompted.
“Once a month!” she exclaims, in a tone of outrage, as if I’ve told her she can expect a monthly beating starting in a few years.
“Is it a tradition,” she enunciates the word carefully, and I wonder if it’s one they’ve been using in kindergarten this week, “or does it just happen?”
“A tradition? Uh, no, it’s not a, a tradition.” 
“Dang it, if it was a tradition I wouldn’t have to do it!” she exclaims.
I am just thinking to myself that I love this conception of what tradition means (a thing you don’t have to do) when I observe that she is now (in illustration of the counterfactual “it” she wouldn’t have to do if periods were a tradition? In illustration of what it is that “just happens”?) gleefully plucking an invisible knife out of the air and miming stabbing herself in her crotch.
“Stab!” she yells, jubilantly, as she plunges the invisible knife into her groin.
I give an involuntary yelp.
“Yikes, no. Jesus.” I am actually wincing. “No. No. There’s no stabbing.”
As she turns away I find myself wondering if she will perform this mime in front of other people and if it will somehow be traced back to my explanation of what periods are.
“There’s no stabbing!” I call after her, as she walks away, and I think to myself that this is what is called losing control of the narrative.
 Although it kind of is, isn’t it? Which is to say, a menstrual period is both something that “just happens” (or, sometimes, sometimes momentously, doesn’t happen) and a “tradition.” I remember when I was on the pill how weird I thought it was when I realized that some of the pills were placebos so as to create an artificial period. And that the reason for this was because the inventors of the pill “believed that women would find the continuation of their monthly bleeding reassuring” (quoting from Malcolm Gladwell’s 2000 New Yorker article, “John Rock’s Error.”)