Unlike with most of the important changes I go through, I can pinpoint the exact moment when I stopped grieving a recent traumatic event in my son’s life. This unexpected shift happened during one of those crazy-hot days we had in April. I had taken my son, Nat, home with me for the afternoon — he lives in a group home with other intellectually disabled adults. He’s supposed to stay at the home on weekends, to get used to this new house, to become independent of us. But on that sunny Sunday, I just wanted him with me.

On our drive to our house, I realized that I had no plans for him. That’s usually the case, though, and most of the time it weighs heavily on me. It is hard to know what to do with Nat because most of his happiness comes from within, and from unknowable things — not from engaging with others. Instead, he prefers to sit quietly in a sunny spot and talk to himself in his own language. I think that talking to others is Nat’s hardest task, and I believe he invented his “Silly Talk” — at the age of 5 — as a way to keep our words out. People see Nat chattering these apparently nonsensical words, and they conclude that he can’t understand them, that they shouldn’t even try to talk to him.

He cannot tell me so many things. He could not even tell me that his ribs were broken last summer, or how it happened. I found out when I saw the fist-shaped bruise on his chest. And I have been grieving, beating myself up ever since because I did not know, and because I failed to protect him. I could not stop feeling this way because I am his mother and somehow should have known.

I didn’t want to just sit around with those sad feelings on such a beautiful day, so I opted for the easiest solution: a trip to Starbucks. It is an easy thing to do with him. Going for treats is something we both like. It would get us outdoors, and I would get an iced coffee out of it. And of course Nat would have his favorite cookie — “chaw-chih coogie” is how he pronounces it.

Nat can order a chocolate chip cookie. Just like he can tie his shoes, or step on a scale at the doctor’s office. But still the nurse or the person at the counter speaks to me as if he is not even there. When they do speak to him, it’s “good boy!” He’s 27.

So when we got into line at Starbucks that afternoon and waited our turn, I was prepared for the usual twinge, the reminder of the chasm between Nat and the world. That, on every level, he is a stranger to most human beings.

The barista, a slim, pale young man with brown hair, looked over and asked what he could get us. Nat said, “Chaw-chih coogie,” and I got ready to translate, embarrassed for Nat, and for the barista. I was about to step in and help, when in that split second, I don’t know why, I just looked away. I could not do it. After all, Nat had told the guy loud and — well, kind of, clear. I waited.

So the barista simply repeated to Nat, “Chocolate chip cookie?” I glanced at Nat, my mouth still shut.

Maybe he sensed the guy was actually taking him seriously. Maybe he noticed I was holding back. Whatever it was, Nat answered him with a perfect, soft, “Yes.”

And off the guy went. Just like that, like nothing at all had happened. “Oh, you want it warmed up?” he shouted.

“Yes,” Nat said, again.

No one was even looking, no one cared. Why should they? It was just a guy buying a cookie. But to me Nat was like Abraham, stepping forward and saying to God, “Here I am.”

I fished a dollar out of my wallet and stuffed it in the tip jar, my meager offering of thanks. Nat collected his cookie and found us seats at the window. I floated my way over to him, so light, so proud. And hopeful. I hadn’t felt that way with Nat for so long. We sat side by side with too much sun in our eyes, not talking, because everything important had already been said.

Susan Senator of Brookline, Massachusetts, is author of “Autism Adulthood: Strategies and Insights for a Fulfilling Life.” She wrote this for The Washington Post.