Pi is famously calculated to trillions of digits—but how many of them do we really, really need? This video demonstrates that, actually, just 39 will do.
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Taking pi to 39 digits allows you to measure the circumference of the observable universe to within the width of a single hydrogen atom. Sure, you could use more, but it's not really worth the effort, and that means that on a day-to-day basis, you can ignore far more of them. [YouTube]