It has attractive clear illustrations of one animal per page, a simple repeating structure with only a few new words each page, nice rhyme/meter. Its structure involves repeatedly anticipating the next animal, which makes it easy to memorize.
It is a book my kids enjoyed “reading” aloud to themselves at age 1.5–2 before they could fully remember the content of more complicated picture books.
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Of course, this isn’t the only type of book small kids should be listening to, and in the genre of extremely simple picture books there is a lot of abject garbage.