Elon Musk says

“I've said many times: prototypes are easy, production is hard.”

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“Measuring programming progress by lines of code is like measuring aircraft building progress by weight.”

“If you give people tools, and they use their natural abilities and their curiosity, they will develop things in ways that will surprise you very much beyond what you might have expected.”

“Most people overestimate what they can accomplish in a day - and overestimate what they can accomplish in a year.”