According to one of his former coworkers in a new documentary that provides us with an inside look at the Tesla CEO, Elon Musk used to become quite irritated when employees in his first firm Zip2 didn’t stay late before he amassed an estimated net worth of $238 billion.

Jim Ambras, a former vice president of Zip2, discussed his experiences in the opening segment of a BBC documentary The Elon Musk Show. Ambras added that Musk would deliberately walk through the offices in the late evening hours to check on who was still there and who had left. Musk would become so enraged that “he went red” if the entire company wasn’t still there at 9:00 pm.

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Systems engineer Branded Spikes, a different former employee at Zip2, reportedly discussed his old boss with BBC producers, according to Business Insider. Elon Musk was referred to by Spikes as a “strange person” who worked long hours, slept beneath his desk, and seemed to have all the answers. As his mother Maye Musk admitted, she struggled to wake Musk up in the morning because he would stay up all night reading books. This suggests that Musk’s tendency of working late started as a young boy.

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Based in Palo Alto Elon Musk and his brother Kimbal Musk established Zip2 in 1995 as a city guide software provider for newspapers. In 1999, Compaq paid $307 million to acquire Zip2, and Elon and Kimbal each received $22 million as part of the agreement. After founding Paypal, Musk later sold it to eBay for $1.5 billion in 2002. SpaceX, one of his current enterprises along with Tesla, Neuralink, and The Boring Company, was founded by him in the same year as Tesla.

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Interviews with Elon Musk’s friends, family, coworkers, and opponents are used in the BBC documentary to try and understand the man. The Elon Musk Show won’t air outside of the UK because of copyright concerns.