Toward the end of a week at my sister’s cabin in the Sierras, we spent a bucolic afternoon at a nearby alpine lake. A trio of mallard hens swam up to us at the shore, and we fed them our entire bag of peanuts. Make Way for Ducklings has been a favorite of late, so we called them Mrs. Mallard and her two lesbian girlfriends she keeps on the side. The lil’ monkey said, “They’re not flying over Beacon Hill.” Check.

It struck me how ordinary this experience was to her — that animals in the wild should approach us and interact with such comfort. This and any number of other far more unlikely human-animal interactions are commonplace in the books we read to her daily. My beloved and I, on the other hand, were transfixed. Essentially city-slickers, we each of us knew how rare this was. Not the ducks approaching, necessarily — that’s a classic city park kind of experience, which people anticipate with bags of bread crusts, even. But it was the wild beauty all around us, the fact that the lake was essentially deserted this afternoon (all but for one lone fisherman, far down the shore). The sharp, clear, sun hanging lower in the fall sky. The crisp wind. Something about the atmosphere surrounding the ducks’ approach made it feel especially rare.

Of course every distinct moment is especially rare. Each successive one is the only such one there ever has been, or will be. We just tune into some more than others.

I have been attuned on this trip for several reasons, prominent among which is the fact that it will have been our last as a threesome. I watch our daughter knowing that her baby brother, and a whole new experience of her family life, is one short trimester away. We know this, and we convey it to her frequently; we check out “big sister” books from the library. We consult friends and our sterling local online advice network about how to help ease the transition. But really it’s just that her world will shift, and we know that, and she doesn’t, not really, regardless of how often she makes reference to the “brudder” in Mama’s tummy.

Oh, certainly, her world will shift ultimately for the better: we mean for her sibling to be a blessing to her; fates willing, her most enduring, intimate family companion. But that thought is always closely followed by another. My sister intended the same thing: her kids to be best friends, to accompany each other into their older age, helping one another through hardship, like the eventual deaths of their parents. All the things one hopes for, when one brings a second child into a family. No one looks into the future and imagines the death of one of them. Or at least no one in my family did, before Erik.

Here in the mountains, I think of him. And I think of the preciousness, the exquisite rarity of this moment. The ducks now, never to return again like this — not these ducks; some ducks, somewhere, but not these, in this light, at this alpine lake, on this afternoon. The intimacy our daughter has now, with just the two of us and the one of her. If we are fortunate, we’ll be back at this lake again, but it will be different. If we are dizzyingly fortunate, we will go from a threesome to a foursome, and stay that way. At least until my beloved and I find ourselves looking across a breakfast table at one another with a sweet melancholy, one fall, eighteen or so years from now, a couple once again, our nest empty, the first leg of our parental journey behind us.