The culprit: their toddler. Isla’s colorful subtitles led her parents to have her IQ tested in May when she was approaching 2½, the McNabbs told The Washington Post. By the end of the month, they got the results: Isla had scored in the top 1 percent of the population. Her performance qualified her for membership in Mensa, an organization of people who score in the top 2 percent on IQ tests.

That makes her one of the youngest Mensa members in the country. In 2019, American Mensa spokesperson Charles Brown, while speaking about a Texas 2-year-old who became a member of the organization, said the boy was one of three members younger than 4 and one of 56 younger than 6.

“That’s out of 50,000 members,” Brown told WFAA.

As for Isla, her father, Jason McNabb, 43, said there were a few times during the first years of his daughter’s life that the hair on the back of his neck stood up — what he called “creepy moments” that made him think something exceptional was happening.

But they became more than fleeting moments and gut feelings around the time she turned 2 in November. Isla had an affinity for the alphabet and had been sounding out letters on her own. So her mom and dad — an auditor and a dentist, respectively — got her a tablet as a birthday gift. After writing down some letters, Jason demonstrated for Isla how to sound them out. Wondering whether she could piece those letters together, he wrote the word “red.”

“She sounded it out and said, ‘Red,’ ” Jason said.

The McNabbs tried “blue,” followed by “yellow” and then “purple.” Isla got them all. Then her mom thought of one she was sure would stump her daughter: “orange.”

“There’s no way,” Amanda, 38, told The Post, recalling what she was thinking at the time.

“Everything we threw at her, it just seemed like she picked it up right away,” Jason said. “It was incredible.”

After that, the McNabbs kept teaching her new words. Almost always, she could phonetically sound out the letters until she was able to read the word. Her parents began keeping a list. When they started, Isla’s vocabulary was around 100 words. It quickly grew to 200. They stopped counting at 500.

“Now she can just read,” Amanda said.

At a doctor’s appointment in the past couple of months, the McNabbs told the pediatrician that Isla could read. The doctor assumed they meant that she’d memorized stories her parents had told her repeatedly. Jason and Amanda then directed Isla to a poster in the doctor’s office about the danger of leaving babies alone on an exam table. Isla read every word.

“Oh! She can read,” the pediatrician said.

Isla’s learning wasn’t limited to reading, which most children begin to do around 6 or 7. She started counting and then abruptly began doing it backward. She can do simple math, including subtraction. One day, Amanda kept Isla occupied by giving her some crayons and an empty Amazon box. She noticed Isla had written M-O-M — or a solid approximation, given her lack of motor skills. Reading had evolved into writing.

Amanda researched psychologists who administered IQ tests and found one in Lexington, a little more than an hour’s drive from their home. He told her that he normally didn’t test children younger than 4. But, intrigued by her claims, he made an exception. Isla’s results on the Stanford-Binet Intelligence Scales, which were reviewed by The Post, rate her as “superior” or “very superior” in all categories.

She ranked in the 99th percentile.

Although she’s intelligent, Isla is also a normal toddler. She likes “Bluey,” an Australian cartoon about a blue heeler cattle dog, and “Blippi,” a children’s show on YouTube, which her father described as a “modern-day Pee-wee Herman.” Last month, she started going to preschool and has become obsessed with making friends and with her teacher, Miss Abigail. She also enjoys doing jigsaw puzzles and playing outside.

“Normal kid stuff,” her mother said, adding that Isla is, of course, a fan of reading and the library.

A couple of Isla’s favorite books: the Pete the Cat series and “Chicka Chicka Boom Boom,” which teaches children the alphabet. It does so by telling a story about a coconut tree that collapses after all 26 letters climb it.

Isla re-creates the book’s climax by stuffing her block letters into Booger’s cat tree and then knocking it over to make the letters scatter, her parents said. “We do that multiple times a day,” Amanda added with the exasperated tone and gaze unique to the parents of toddlers.

Her parents are, well, tired. Initially worried that Isla wasn’t sleeping enough, the psychologist who tested her informed the McNabbs that it’s normal for highly intelligent children to sleep less. They were relieved their daughter was healthy but not excited about the prospect of her continuing to rouse them at 4 a.m.

“That’s kind of disheartening for us,” her mother said.

Amanda said she’s sure of one thing. While she was eager to get her daughter tested and excited about the results, she will not be following suit. “I tell people that I’m not going to get tested,” she said.

“I can’t have her knowing that she’s smarter than me.”