Internships remain a crucial way to advance your career, according to the National Association of Colleges and Employers (NACE). In fact, hiring managers say that interning within your preferred industry is more important than your college major or GPA, NACE’s 2018 Job Outlook survey finds. “Employers prefer work experience, and in particular they prefer that the experience was obtained through an internship or co-op experience,” NACE research manager Andrea Koncz tells CNBC Make It.

Even if the internship isn’t directly tied to your dream job, it provides work experience that may help you understand what is most fulfilling to you, Koncz adds. That was the case for Elon Musk. Before he became a billionaire tech mogul, Musk got a taste of professional life as an intern for various employers, including the Bank of Nova Scotia and Rocket Science Games.

Here’s how his internships helped him figure out what he wanted to do.

Bank of Nova Scotia

While studying physics at Queen’s University in Canada during the early 1990s, Musk and his brother Kimbal cold-called numerous people they wanted to meet, including Peter Nicholson, a top executive at the Bank of Nova Scotia. Nicholson “was not in the habit of getting out-of-the-blue requests,” according to Ashlee Vance’s book “Elon Musk,” and he was impressed. As he put it, “I was perfectly prepared to have lunch with a couple of kids that had that kind of gumption.”

Nicholson gave Musk a summer internship at the bank, where he earned about $14 an hour and got to pitch new ideas to his boss. For one project, Musk told Vance, he wrote up a proposal for a bond trade that he estimated could be “the biggest opportunity ever, and nobody seemed to realize.” His boss approved the pitch and passed it along to the bank’s CEO but, to Musk’s dismay, the CEO rejected the proposal.

“Later in life, as I competed against the banks, I would think back to this moment, and it gave me confidence. All the bankers did was copy what everyone else did,” Musk told Vance. “If there was a giant pile of gold sitting in the middle of the room and nobody was picking it up, they wouldn’t pick it up either.”

One of Musk’s takeaways was that “bankers are rich and dumb,” he told Vance, and he kept that in mind when, in 1999, Musk launched his finance start up X.com. The site merged with Confinity in 2000 and became PayPal.

Pinnacle Research Institute and Rocket Science Games

In 1994, Musk spent part of his summer in Silicon Valley and held two internships: He worked at an energy storage start-up called Pinnacle Research Institute during the day and at the Palo Alto-based start-up Rocket Science Games at night.

While at one of his internships, Musk watched as a Yellow Pages salesman dropped in and tried to get the company to buy an online listing in addition to its print listing. The salesman fumbled through his pitch, and, as Kimbal remembers it, that prompted his brother to tell him, “These guys don’t know what they are talking about. Maybe this is something we can do.”

Jeff Bezos had , Amazon, in 1994, and Musk couldn’t bear watching other online businesses form without getting in on the action. In 1995, Musk started a Ph.D. in energy physics at Stanford but dropped out after two days. He realized he “didn’t want to do a Ph.D. at Stanford and watch it all happen.”

Instead, he started a Yellow Pages-like online directory with his brother called Zip2. When Compaq bought Zip2 in 1999, Musk made $22 million.

“You can get a doctorate on many things that ultimately do not a have practical bearing on the world,” Musk said. “And I really was just trying to be useful.”