This is a copy of a post, originally written on Medium. I've since rehosted it here. As a corollary to the famous sentiment “the best way to prove mastery over a topic is to be able to teach it to somebody else”, I’d like to add “the best way to prove mastery over a piece of software it to implement it yourself.” I don’t know whether or not that’s true, but I do often enjoy building my own versions of the tools/libraries that I use.