I've found this "story" by Michael Wolfe [1] (warning: Quora link that you may need to open in an incognito window) to be a fun way to explain why accurate software estimation is so difficult.

[1] https://www.quora.com/Why-are-software-development-task-esti...

"My" idea was also inspired by coast lines, in the initial examples for fractals with the English coastline.

And I often use coastlines as an example to explain why software development is fractal. Will use your link in the future, thanks!