 At the back of Why Johnny Can 't Read , Flesch had included 72 lists of words that young children should be able to read , and Spaulding provided Geisel with a similar list . Geisel later told biographers Judith and Neil Morgan that Spaulding had supplied him with a list of 348 words that every six @-@ year @-@ old should know and insisted that the book 's vocabulary be limited to 225 words . However , according to Philip Nel , Geisel gave varying numbers in interviews from 1964 to 1969 . He variously claimed that he could use between 200 and 250 words from a list of between 300 and 400 ; the finished book contains 236 different words . 
