NEW YORK – As a teenager, Tumblr CEO David Karp would canvass the streets of New York City’s Upper West Side, offering to build websites for local businesses. After his freshman year of high school, the precocious, computer-savvy kid decided to drop out altogether to devote more time to his passion for technology.

A few years later, Karp built Tumblr — the wildly popular blogging forum — from his tiny childhood bedroom, hunched over his laptop with bags of Tostitos. And on Saturday, the 26-year-old technology wunderkind returned home to inform his mother that, in a game-changing transaction, Yahoo was buying Tumblr for $1.1 billion.

“There were a few tears and lots of hugs, and a lot of excitement,” said his mother, Barbara Ackerman. “This is something that he built — it’s his baby — and it’s emotional.”

The deal was a transcendent moment for Karp, who created one of the world’s busiest websites. It boasts 75 million daily posts and a user base that’s loyal, young and hip. While Facebook has morphed into a mainstream social network where grandparents talk golf, Tumblr is still that little corner of the Internet where the cool kids hang out.

True to the company’s laid-back, jeans-and-sneakers culture, Karp’s wry sense of humor remained intact Monday morning, when all employees were summoned to a meeting in Tumblr’s New York headquarters. Cognizant of media reports that Tumblr was on the verge of a sale, everyone waited with bated breath as Karp kicked off the meeting with a tongue-in-cheek announcement: It was time to formulate a new “dog policy.”

“We have gone above and beyond with our dog policy,” he told them. “There is now one dog for every five people in the office at Tumblr at any given time. So we are needing to figure out a better bathroom situation.”

With just six years of Tumblr’s existence under his belt, Karp told The Associated Press that he still considers himself to be a green executive. But he’s learning that he can no longer play the same role that he did in the early days, when he spent most of his time writing computer code.

“Now the team that I get to work with writes much better than me,” he said.

Today he’s the “chief motivator, the guy jumping around every week to make sure the employees are excited about the product we are building and the direction we are headed in,” Karp said.

“He doesn’t have a pretentious or egocentric bone in his body,” said Steve Nelson, the head of The Calhoun School, a private institution that Karp attended until the eighth grade. “He doesn’t take himself too seriously.”

Yahoo said Monday that Karp will remain in control of the service in an effort to retain the same “irreverence, wit and commitment to empower creators.”

Tumblr, which will remain based in New York, has a minuscule 175 employees, compared with Yahoo’s 11,300.

Yahoo CEO Marissa Mayer showered praise on her new employee, saying Karp possesses a rare combination of computer programming prowess and a sense of aesthetics.

“He will be one the legends of his generation in terms of an entrepreneur who has really changed the way people express themselves,” Mayer said.

When he was a little boy, Karp showed an early fascination with electronics and liked to pull toys apart and reassemble them, his mother recalled. By age 14, he liked girls and playing video games like other teenagers, but they never fascinated him as much as the promise of computers.

“It was mostly about figuring out how to make computers do the things that I wanted to see them do,” he said.

Though he was enrolled at The Bronx High School of Science, a prestigious New York City public school, attending the school required him to make the long commute to the city’s northernmost borough every day. And he found himself wanting more freedom to explore the digital world on his own.

So his parents let him drop out. Instead, Karp embarked on a homeschooling regimen with the help of several tutors, learned to speak Japanese and pursued his digital interests with abandon.

“He needed the time in the day in order to create,” said Ackerman, who divorced when her son was a teenager. “And that’s what he is. He’s a creator.”

As popular as Tumblr has become, the service remains unprofitable. And the company has had its share of stumbles.

Advertising has been a missing ingredient because Tumblr, like many online services in their early stages, focused on building a loyal audience before turning its attention to making money. That’s why the Yahoo deal is so pivotal for Karp and his small team of creative thinkers.

For now, though, they’re basking in the glow of joining forces with an Internet behemoth. As for Karp’s mother, she was positively giddy at the prospect of meeting Mayer on Monday night.

“I’ll get to talk her ear off about how wonderful David is,” Ackerman said. “That’s what moms do.”