Speaking in an interview with Y Combinator on Thursday, Musk said that his “biggest epiphany” of the year has nothing to do with building cars or getting a rocket to space; it’s all about the process of building those vehicles.

“The biggest epiphany I’ve had this year is that what really matters is the machine that builds the machine, the factory,” he said. “And that is at least two orders of magnitude harder than the vehicle itself.”

Musk’s year has been one dominated by talk of manufacturing.

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In July, Tesla (TSLA) held a multiday event celebrating its Gigafactory, a massive factory under construction in Reno that will produce both battery cells and packs for the company’s upcoming Model 3 car. The vehicle, which was in development for quite some time, will cost $35,000, bucking the trend created by the Tesla Model S and Model X, which both cost tens of thousands of dollars more and are considered luxury vehicles.

The Gigafactory is designed to resemble a chip manufacturing facility rather than a traditional battery plant. In a statement earlier this year, Musk said that manufacturing as a practice “needs love,” and he promised that Tesla would give that to the industry. He’s also been fond of his depiction of manufacturing as “the machine that builds the machine,” the same phrase he used in his interview with Y Combinator.

For Tesla’s business, there is a lot riding on the Gigafactory’s success. The factory is expected to dramatically reduce the cost of producing Tesla’s lithium-ion battery packs, which will allow the Model 3 to get down to an affordable price. If the factory, which will cost at least $5 billion to build and in July was just 14% completed, fails to deliver on that promise, the Model 3 and perhaps Tesla’s own lofty goals for the future could be in doubt.

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“It’s about making enough electric cars, enough stationary battery packs that it actually moves the needle from a global carbon production perspective—that it actually does really change the world,” Musk said earlier this year, explaining the purpose behind the Gigafactory. “It has to be big because the world is big.”

Still, Musk acknowledging that building a factory—something many take for granted in technology production—is his hardest work this year is notable. Musk, after all, is leading a major electric car company that’s trying to compete with massive and entrenched competitors like General Motors (GM). He’s also at the helm of SpaceX, where his company is trying to improve space travel.

Judging by what Musk says, the moonshot isn’t necessarily what keeps him up at night; it’s producing the moonshot that does it.