IN 2005, Randall Munroe, a young NASA robotics expert, posted to the web a comic about the mystical number pi. Among the infinite string of digits were letters that spelled out, “Help I’m trapped in a universe factory.”

His pi comic spread quickly, and nearly a decade later Mr. Munroe sits atop a small empire built around xkcd, an online comic revered by science students, computer programmers and Silicon Valley workers.

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