Chicago writer Robert Hughes is the parent of a child with special needs. And there are times that he spends with his son, Walker, where the simplest things, bring a smile to both their faces.

We are two tall men crossing a busy Chicago intersection, holding hands.

I am 61, and with my aging face and bald head, I look every inch the baby boomer. Walker, my 25-year-old, quite low-functioning autistic son, looks like a handsome young quarterback. One day at this spot—a shining moment for me—a panhandler thought he saw a May-December gay couple approaching him and muttered something like “ah, young love.” I could have shaken his hand.

Usually we are not mistaken for normal-spectrum city pedestrians. As habitual long-distance North Side hikers, we are hard to miss. Walker sends signals even a block away that a different sort of person is at large. He walks-nearly runs-with a forward-leaning bounce that seems a little too energetic, too enthusiastic really, for a Sunday morning. He grabs my hand and holds it for several seconds, for no apparent reason, then lets it go. He stops and turns and stares at something in the distance while I turn too and try to figure out what he’s looking at. He does little spins before sitting on a bench, makes sure every turn we make is 90 degrees (no shortcuts, please!), and speaks in loud telegraphic bursts: “Borders hot chocolate!”

But the main feature, the one that would give him away in a science fiction film about aliens among us, is his smile.

It’s marvelously inappropriate.

For one thing, unlike mine, it’s perpetual. He greets any kind of day—overcast, stormy, hot, freezing, lake-effect blizzardy—like it’s the best thing that’s happened, ever. He tilts his head and gazes upward as if food rations were parachuting from the sky.

For another, his smile radiates meaning. It shows amusement, joy, surprise, teasing affection, charm, sometimes all at the same time. When I chatter at him about things I know he knows like songs and friends, videos and sights along the way, he looks at me as if to say what great in-jokes we share.

And most of all, it’s infectious. All of us check out anybody who’s breaking the rules of sidewalk culture, who moves or talks or seems a tiny bit over the edge. We could never articulate what this edge is, where the boundaries of the pedestrian normal are, but we know when anyone veers outside them. Years ago, this look from passersby drove me crazy, but having grown up a bit after a few thousand treks with him, it now just amuses me. For I’ve come to know that they can’t help it. And seeing his face, they catch his smile and they smile too. They can’t help that either.

I think they see what I see: a young man with a problem but with a magical knack for appreciating life—and a rare, rare gift for passing along this delight to others.

Robert Hughes is the author of Running With Walker: A Memoir and teaches English at Truman College in Chicago.