“You walk one mile south, one mile west, and one mile north…”

• Elon Musk reportedly asks job candidates to solve a tricky riddle during interviews at SpaceX.
• Most people eventually solve the riddle. Can you?
• Once you’re finished, try your hand at our other challenging brain-teasers for adults.

You never know what to expect from Elon Musk, whether he’s saying settlers will die on Mars, claiming the sun will someday power all of civilization, or naming his new baby after a spy plane. (And that’s just in the last few months.) So it may come as no surprise that interviewing for a job with the Tesla and SpaceX CEO may involve, well, a few surprises.

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According to the 2017 Musk-authorized biography Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future, there’s at least one question that Musk supposedly puts to prospective employees to catch them off-guard.

As author Ashlee Vance puts it:

“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?”

When SpaceX was first hiring, Vance writes, Musk would personally interview each employee—and later, each engineer as the company grew larger. To their credit, most of them would generally guess the correct answer: the North Pole. That’s when Musk would hit them with another surprise follow-up question: ”Where else could you be?”

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The twist is that the riddle actually has two correct answers.

This fast-paced 3-D puzzle game involves a combination of quick thinking, logic, and luck to stack your spheres to earn the most points.

“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,” Vance writes.

He adds, “[Musk] 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.”

Elon Musk: The Popular Mechanics Interview

For a more in-depth explanation of Musk’s favorite riddle, check out this detailed breakdown from mathematician Presh Talwalkar, the man behind the popular math and science blog MindYourDecisions.

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➕ Plus: Want more riddles? Try your hand at one of our challenging riddles of the week. No cheating!