After hearing another father in a store explaining Down syndrome to his son, Robb Scott made this video to spread awareness. Scott's son has Down syndrome, and he said "it's fun, it's brilliant, it's amazing...it's not an illness." (Robb Scott)

During a recent trip to a store, Robb Scott overheard a little boy ask his father a simple question:

"What is Down syndrome?"

The boy's father told his son that it's an illness of "not knowing anything."

Scott — the father of a 5-year-old with Down syndrome — wanted to correct the man's statement; but, for reasons he can't quite explain, he hesitated.

By the time the 41-year-old dad from Truro, Nova Scotia, left the store and got into his car, he'd decided that he couldn't live with his decision to avoid speaking up. So he whipped out his smartphone and recorded an tearful, five-minute video that's gone viral since it was posted on Saturday.

The video begins with Scott recounting his experience inside the store before he explains his belief that Down syndrome isn't an illness.

"Down syndrome is literally one of the most beautiful things that has ever happened in my life," he tells the camera. "It's fun, it's brilliant, it's amazing, it's funny, it's kind, it's loving, it's cuddly."

His son, Turner, has Down syndrome, Scott said.

"They're great teachers, people with Down syndrome," he added, his eyes growing watery. "It's not an illness, it's not an illness. It is not ... even a disability."

No two people with Down syndrome are alike, according to the U.S. National Library of Medicine. The condition — which occurs when someone is born with an extra copy of chromosome 21 — is often accompanied by physical and intellectual challenges, such as a smaller statures, health problems and learning disabilities, according to the NLM.

In the video, Scott notes that he almost approached the father in the store to address his comment directly. He didn't perceive what the man said as being mean-spirited, but instead based in ignorance. Ultimately, what seemed to trouble Scott the most was letting a moment pass by that could've been used to educate a younger generation, Scott told ABC News.

"This child was genuinely asking, 'What's Down syndrome?' — and this child is a blank slate," Scott said. "I let him understand it for something it wasn't. I let his father define it for him and that hit me hard. This was a child — a child that's my sons age; and I could've corrected him, not in a rude way, and I didn't."

As he reflects on the missed teachable moment, Scott said he was compelled to atone for his indecision and "karmically reset what just happened.”

Near the end of  his video, he challenges the idea that people who read slower or can't run as fast have a disability.

Down syndrome, he says, is "the best thing that ever happened to me."

"I believe people are teachers and learners," he said. "We're both. We have the ability to teach things, and we're here to learn things. A well-educated man does not have more to teach than my son. He has different things to teach, but he does not have more to teach."

Scott told The Washington Post on Thursday night that he never expected the video to explode in popularity; but he's been heartened by the response.

"If 10 people heard it, then I could feel better," he said. "I thought maybe five or 10 people might share the video. But what's happened has really been astonishing. I'm still at a loss for words why this video seems to have struck such a deep chord."

He added: "I feel like 1.5 million people have told me 'It's okay.'"

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