Last week we went to the classroom, my son and I, and looked at the boxes. The school has taken on new buildings and so there were a lot of boxes, most of them stacked against the wall and labeled in crayon along the side. SCIENCE LAB, they said. CONSTRUCTION PAPER. PASTELS.

In the classroom it was entirely quiet. The walls were still bare and the short yellow chairs were piled up in one corner and my son, who sometimes declines to these days, held my hand. We had been at the school picnic in the park nearby and he had talked to a boy named Miguel, who was shy, which I suspect made my son feel vigorous and tough. My son is 5 years old. He is not very tough at all. Tomorrow he goes to kindergarten.

Tomorrow, that is, my son commences elementary school. I am practicing saying this. He will ride the school bus. All across the land there are extremely small persons climbing this week onto buses and bicycles and the front seats of parent-chauffeured automobiles, and Aaron Joseph Sokol -- I will get used to saying this -- is one of them. He's pretty tall, and he likes to sing really loudly when he gets to the chorus of "I've Been Working on the Railroad," and he needs new shoes. If I weep, on my honor, I will leave the premises first.

Here is one of the foolish assumptions of the modern age: When you have sent your child to day-care centers and nursery school, the next step up is no big deal. The boy already has a lunch box, after all; he knows about hanging his jacket in the cubby with his name on it, and by the age of 3 he had developed sufficient social graces to pour his own apple juice and refrain from snatching the celery stick from his table companion's hands.

The rigors of the passage, I imagined, would take no toll on me. I am a veteran of child-care cooperatives and potluck fundraisers and car-pool arrangements that would make a World War II field marshal retreat to his tent. I have survived the Day-Care Workers Who Cannot Stand Each Other crisis, the Community Center Board Firing Its Fourth Preschool Director crisis, and that memorable 5:22 p.m. epiphany we recall these days -- actually, we try whenever possible not to recall it at all -- as I Thought You Were Picking Them Up.

So I was cool, okay? I took Aaron to buy a notebook. The kindergarten teacher had sent him a nice little note asking him to bring 10 buttons on the first day of school, and since we are all busy frowning at excessively academic kindergartens, I'm sure 10 buttons are fine things to have when you report to class. But the buttons are down at the uninteresting end of the store. It was necessary that my child and I contemplate the notebooks, which were piled in the ancient end-of-August manner, the pencil cases and binders piled higher alongside them. We stood there and studied the array, and I want you to know that I was still a model of self-composure -- PeeChee folders, I observed, still have the same brown-tone football player lunging blindly across the front.

Aaron found an exemplary binder, with Velcro tabs and pockets in the ring dividers and a complicated sheet up front that says in large letters, "DATA PLANNER." I gave him some money to pay for it. He held his new binder under one arm. "Can I have a cookie now?" he said.

And still I was cool.

And then, in Ari Krupnick's bedroom, he lost his first tooth.

A major search was launched, to no avail, and when it was over Aaron and Ari reported to our front door and grinned, the way they do. The brown-eyed one had all his teeth and the blue-eyed one had a single bloodied gap and I looked at them both and felt something fierce around the edges of the heart because this, after all, is the place of the real parting -- the place of the growing away.

Aaron is going to one kindergarten. Ari is going to another. Neither of them commands any working memory of daily life without the other; they are best friends, and have been since a long time before they had words. At the small child-care center where they met, if that is the verb you can use for persons not yet old enough to sit up by themselves, the young woman in charge told us that one would wake from his nap and belly-hump instantly to the other's sleeping mat. Their parents came and went in the complicated way working parents do, but Aaron and Ari learned together how to march and make stegosaurus noises and assume the voice and body stance of He-Man, Defender of the Universe. When they were large and grown-up and given serious nursery school assignments like the drawing of family trees, Aaron made out his list with some care and pasted Ari's picture directly between Daddy and Grandma Liz.

"Okol," I believe, was how Ari first managed the name. Where was Okol, could Okol come to play, how was he to sleep without Okol nearby? And there were we, Okol's hapless parents, wrestling eight blocks away with a 2-year-old boy whose principal response to household crises was to drop to the floor and wail, "I want Ari," as though the world was a place made bearable only through the graces of his small best friend.

Quite possibly he was right about that. I have watched Aaron and Ari from the other side of the room and remembered the way the outside closed in on me and the first person to become my best friend; she had a closet, an odd one so deep that you could crawl past the clothing and sit together in the dark space inside. Aaron and Ari had a tent. The tent looked like a jeep and sat on top of Aaron's bed -- it was the only stab I ever made at what might wildly be referred to as children's interior decoration -- and they would disappear into its midst for long periods of time, reaching out now and then to make sure the zipper was not sliding up and forcing them to face the light of day.

They didn't need the tent, of course. I once spent nearly an hour watching Aaron and Ari and a miscoiled metal spring, which they wrapped around themselves, and shouted into, and took turns throwing erasers through. They scarcely needed adults, for that matter, except to turn on the bathtub and reach for the peanut butter; I once tried to console Ari, after he had whomped his elbow hard enough to make him cry, and Aaron led me kindly but firmly to one side before he took things in hand himself.

I wish I could tell you what he said, but it was not in a language I have ever heard before, and it made Ari laugh so hard he practically fell off the curb. My son told me once that Ari was the leader of his heart. He was nearly asleep when he told me, but I knew that it was true, and I know also that if he has a bad moment this week and looks desperately around his classroom in the way we all do sometimes when things are too new and peculiar to bear, it will not be his mother he is looking for.

I heard them in the bathroom last week, and they were talking about kindergarten. They were sitting on the bath mat, Ari's dark thick hair barely visible through the doorway, and you could see that they were worrying it through, bracing each other up.

"My kindergarten has a whole room for science," Aaron said.

"My kindergarten has two teachers, and they're really nice," Ari said.

Their talk was clearly private and I moved away to let them be, but I was all banged up with love and pride and impending loss, so I took them out for ice cream bars. They ran most of the way. They are 5 and it is September and there is nothing I can do about it, but I think now that this at least is something that will not go away, these two boys running, the one windmilling his arms, the other shouting into the air. The leaves are underfoot. The driveways make excellent cover. Where are those boys, I am supposed to cry, as I pass with the baby stroller before me, have they taken a taxi to Mexico? When they burst into view, shouting their terrible boos and laughing like maniacs, the baby shrieks and flings both hands into the air. The baby is blond and wild and wants to climb from her stroller. I think I know what the baby is after because I have felt it myself, watching those boys, watching sometimes what they have and sometimes what I see that they will have to lose. They will not lose it all and maybe they will not even lose most of it, but the boys are going to school, real school, separate schools, schools with science labs and tall gangly children and teachers who take attendance in the morning, and nothing for those boys will ever be quite the same again.