I’ve had this thought often: When my kids were very little, I knew them by heart. I assumed that was true because back then I was with them 24/7. As a stay-at-home mom, I was in the frame of virtually every moment of their lives. I knew the backstory of every smile, the cause of every tear. In any given day I knew everything they ate, how long they’d slept, and what had made them fuss or cling. I knew how their skin would smell after a day at the park, and the exact weight of their tiny bodies on my lap as I read to them at night (while secretly taking hits of the lavender scent coming off the tops of their freshly bathed heads).

I knew what books to read, their favorite toys, the games they didn’t like. I knew which stuffed animals they couldn’t live without, and where to find them when they’d gone missing.

It dissipated gradually, this knowing. Their stuffed animal collection grew beyond the point at which I could remember all the names of their furry friends. They started making up their own games, and wanted to play by themselves. They learned to read, taking stories into their young minds all on their own. And I was so grateful for those bits of reclaimed time—a few minutes to shower in peace, to scroll Facebook without interruption—that at first I didn’t realize what I was losing.

And then it was gone. Both of them started school, and hourslong chunks of their days were lost to me. I got divorced, and three- and four-day stretches became childless black holes. I’d sit on my sofa alone and weep over the not-knowing, the not-being there. (Sometimes, as silly as it might seem, I still do.)

As they stretched and grew away from me, I had to work harder at knowing them. I had to ask the right questions at the right times, take my openings when they came. We’re still very close, them and me. And until recently I comforted myself with the nostalgic notion that part of why I know them well now is because they still are, at their core, the unique humans I catalogued so completely years ago.

But a couple weeks ago I came across some old video clips of them. While the current teen versions of my kids played video games in the other room, I sat at my computer and watched their small selves play toy instruments and toddle around the various living rooms of our past. And a new kind of knowing dawned: I did not know those versions by heart anymore. Not even close.

It was sort of surreal, watching two people I used to know very well but haven’t been around in more than a decade. Both of their little-kid voices sounded familiar, but the unique inflections and toddler phrasings were a surprise, not a knowing. I saw toys, felt emotions, in these rooms of our past that I’d completely forgotten existed.

What would it feel like to hug them right now? I wondered. But I couldn’t bring that feeling back. It was like trying to sing a song you love but can’t quite recall when you desperately want to; the lyrics and melody tickle the edges of your conscious mind but keep drifting, maddeningly, to the depths of your memory the more you strain to bring them to the surface.

I chuckled at the way Evan danced so maniacally, smiled as Kostyn ran around and around in circles. It seemed preposterous that I live with the two kids I was watching on the screen. Kostyn in particular is so wildly different from their toddler self—the carefree rambunctiousness given way to more deliberate, deep-thinking sensibilities.

I have changed, too. I imagined walking into those rooms as I am today, and wondered how they would react. Would they run to me? Not likely. I look different and feel different than I did then. Not drastically, but they’d notice. I’ve aged inside and out. And my lifestyle and perspectives are so different now, I bet the energy I give off has been equally transformed into something completely foreign to their 2010/11 selves.

Studying the videos made me wonder how many versions of ourselves we live and leave behind. Does that shift happen in a year? a moment? a milestone? Does it happen with every conversation, with every new favorite song? What makes us “us” within the skins, beliefs, and pronouns we outgrow and shed?

I sat for a while disoriented, then just aching with love for those two tender souls who are not “mine” to know in the first place. I thought about a morning several months ago, when Kostyn, Evan, and I were crammed into the lone bathroom in our house, all getting ready for the day. As I brushed my teeth, I took in the three of us in the mirror, each focused on our own reflection. No one was talking, and the silence made me realize that each of us was, in that moment, hearing a different voice that the others couldn’t hear. In that toothpaste-splattered mirror, the terrible, magical truth dawned on me that my kids have internal voices and narratives that I have never heard, and inner lives that I will never know.

Oh god. I know my own inner voice too well; my eyes grew wide and worried in the mirror, and I had to leave the room. I stood in the hallway for a moment, half-praying: I hope they are kind to themselves. I was still just a few feet from them, but I felt far away, a murky chasm of unknowing between us that can’t ever be crossed. It made me sad.

But it’s OK, right? The truth is I have never known my kids by heart. At any given time, a momentary version of me understands as deeply as I can the accessible parts of a momentary version of them. One of us is always on the brink of rounding a corner, turning a page to unveil a new model of ourselves, the latest upgrade. Slightly taller (or shorter?!). A touch more (or less) expressive. A bit more bruised, resilient, hopeful.

How great that we can get to know the people we love again and again and again. A trillion Kostyns and a trillion Evans coming in and out of focus over the course of a lifetime. A zillion hellos and silent goodbyes of our changing selves, all strung together with inside jokes, memories, and snapshots; stacked piles of outgrown clothes waiting for the next trip to Goodwill; fuzzy digital files of us dancing to songs we haven’t listened to in ages; and enduring love that carries us from beginning to end.