From Elon Musk, by Ashlee Vance, pg 220-222:
Like many tech companies, SpaceX subjects potential hires to a gauntlet of interviews and tests. Some of the interviews are easygoing chats in which both parties get to feel each other out; others are filled with quizzes that can be quite hard. Engineers tend to face the most rigorous interrogations, although business types and salesmen are made to suffer, too. Coders who expect to pass through standard challenges have rude awakenings. Companies will typically challenge software developers on the spot by asking them to solve problems that require a couple of dozen lines of code. The standard SpaceX problem requires five hundred or more lines of code. All potential employees who make their way to the end of the interview process then handle one more task. They’re asked to write an essay for Musk about why they want to work at SpaceX.
The reward for solving the puzzles, acting clever in interviews, and penning up a good essay is a meeting with Musk. He interviewed almost every one of SpaceX’s first one thousand hires, including the janitors and technicians, and has continued to interview the engineers as the company’s workforce swelled. Each employee receives a warning before going to meet with Musk. The interview, he or she is told, could last anywhere from thirty seconds to fifteen minutes. Elon will likely keep on writing e- mails and working during the initial part of the interview and not speak much. Don’t panic. That’s normal. Eventually, he will turn around in his chair to face you. Even then, though, he might not make actual eye contact with you or fully acknowledge your presence. Don’t panic. That’s normal. In due course, he will speak to you.
From that point, the tales of engineers who have interviewed with Musk run the gamut from torturous experiences to the sublime. He might ask one question or he might ask several. You can be sure, though, that he will roll out the Riddle: “You’re standing on the surface of the Earth. You walk one mile south, one mile west, and one mile north. You end up exactly where you started. Where are you?” One answer to that is the North Pole, and most of the engineers get it right away. That’s when Musk will follow with “Where else could you be?” The other answer is somewhere close to the South Pole where, if you walk one mile south, the circumference of the Earth becomes one mile. Fewer engineers get this answer, and Musk will happily walk them through that riddle and others and cite any relevant equations during his explanations. He tends to care less about whether or not the person gets the answer than about how they describe the problem and their approach to solving it.