The Microsoft cofounder had an appetite for computer coding as a teen—when he was 13, he started sneaking out of the house to learn the craft. In his new memoir, Source Code: My Beginnings, Gates recalls that his house didn’t have a computer (like many others at the time), so he had to get crafty. He started slipping away at night, often returning home at 2 a.m., unbeknownst to his parents, who were too busy being attorneys and civic activists to notice. Taking a 20-minute bus ride across town, he got access to a computer and practiced code. 

“I certainly benefited from a little bit of laissez-faire treatment,” he wrote. 

Passersby in the Seattle area were also unfazed by a 13-year-old roaming the streets in the wee hours: “No one seemed to wonder why a kid was out alone at that hour.”

A local company, Computer Center Corp., gave unlimited screen time to Gates and his peers at Seattle’s Lakeside School computer club in exchange for their services. Gates could stay until the early hours writing and testing his bespoke code, and in return, he would smooth out bugs in the company’s programming.

If it weren’t for these crucial hours and learning opportunities, Gates wrote, he might have missed out on the stroke of genius to write software for personal computers. Microsoft might have never existed—or it wouldn’t be the $3.1 trillion business it is today. 

“We were kids…none of us had any real computer experience,” Gates wrote. “Without that lucky break of free computer time—call it my first 500 hours—the next 9,500 hours might not have happened at all.”

And Gates’ rebellious streak at 13 was just the beginning. Even at the most prestigious college in the U.S., he couldn’t just follow the beaten path.

Dropping out of Harvard to seize the moment

Gates enrolled at Harvard University in 1973, but only three semesters later, the then-20-year-old entrepreneur dropped out of the Ivy League to chase his dreams. 

“I had to give in to the inevitable, and give up school and, of course, never go back,” Gates told CNBC.

Gates was waiting for the right moment to make his break—and it came when his Microsoft cofounder, Paul Allen, rushed into his room holding up an issue of Popular Electronics magazine. The cover featured the “world’s first minicomputer kit to rival commercial models.” The computer, dubbed the Altair 8800, was made by Micro Instrumentation and Telemetry Systems (MITS). The pair saw an opening, and decided to seize the moment. 

Read more from Fortune

“Until then, we felt like, ‘Hey, this is going to happen and we’ll figure out our timing and what kind of company to do,’” Gates said in the CNBC interview. “The panic about, ‘God, it’s happening without us,’ was when Popular Electronics had the kit computer [on its cover]. Little did we know, [MITS] basically hadn’t assembled any of them.”

Although Gates dropped out of Harvard to become a pioneer in the industry, it wasn’t an easy decision. He described enjoying the college’s classes and being around smart people. He had some friends—not too many—who made him feel comfortable. Gates attempted to return to the Ivy League institution for two more semesters a few years later, in 1976. But it wasn’t compatible with his new career.

In the end, Gates admitted that if he had finished his degree before launching Microsoft, he might have missed his window of opportunity. 

“It was good to be early,” he said. “We learned a lot [about how to run a business], and we moved up to Seattle, where it was easier for us to hire [the best software engineers].”