An XKCD cartoon better explained by Explain XKCD here: https://www.explainxkcd.com/wiki/index.php/936:_Password_Strength The short version is we have been trained to create passwords that are very hard to memorize with relatively low bits of entropy, so a machine can guess them relatively quickly, whereas a long statement even when using only plain characters such as "correct horse battery staple" makes such an impression that we memorize it almost instantly and provides far more bits of entropy despite being only plain characters, thus making it much harder for a machine to guess. Sadly website developers do not realize or understand this.
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