Engineer and tech entrepreneur Elon Musk of The Boring Company talks about constructing a high speed transit tunnel at Block 37 during a news conference on June 14, 2018 in Chicago, Illinois.

Tesla CEO Elon Musk has had a busy year keeping up with the electric car company's production and the resulting challenges. Even though Musk has pulled all-nighters in the Tesla factory, he still finds his work fun because he gets to accomplish what he first set out to do: be useful to other people. "It's actually pretty fun building electric cars. Product is useful to other people and good for environment," Musk tweeted on Thursday. "I think that's fundamental goodness." Elon Musk tweet When Musk was 20 years old and studying physics and economics at the University of Pennsylvania, he focused on five big ideas: the internet, sustainable energy, interplanetary travel, genetics and artificial intelligence. Since then, Musk, 46, has founded and led companies that revolutionized each of those areas. The underlying question behind all of Musk's business ventures seems to be: "What can I do that would actually be useful?" "If somebody is doing something that is useful to the rest of society, I think that's a good thing," Musk told Y Combinator CEO Sam Altman in a 2016 interview. "It doesn't have to change the world."

To be successful, it's important to measure the impact you will have on others, Musk argued. "That's why I think having something that makes a big difference, but affects a sort of small-to-moderate number of people is great," he said. In college, he explained, the first task he wanted to tackle was electric car innovations. After moving to Stanford to get a Ph.D. in energy physics, Musk got a summer internship at an energy storage start-up called Pinnacle Research. "That's actually what I worked on as an intern, [it] was advanced ultra-capacitors, to see if there would be a breakthrough relative to batteries for energy storage in cars," he said. But Musk dropped out of Stanford in 1995 with $110,000 in college debt to do something he thought was more useful: go start an internet company. The result, Zip2, aimed to make media companies like the New York Times and Hearst go digital. For perspective, Amazon founder Jeff Bezos had just launched his online bookstore in 1994. Maye Musk tweet Musk couldn't bear watching online businesses boom without getting in on the action: "I didn't want to do a Ph.D. at Stanford and watch it all happen," he said. "You can get a doctorate on many things that ultimately do not a have practical bearing on the world," Musk added. "And I really was just trying to be useful." Shortly afterward, in 1999, he co-founded online financial services company X.com, which a year later merged with entrepreneur Peter Thiel's own financial site to become PayPal. By 2001, Musk left the company, but he made millions when PayPal sold to eBay. Several years later, when he explained his plans to colonize Mars and improve high-speed travel around earth with SpaceX, Musk said plenty of people thought his ideas were "crazy" — and "they were not shy in saying that." Even Musk thought his idea was crazy, but as he noted, technology "only gets better if smart people work like crazy to make it better."