She had no idea she would one day become one of the Holocaust's most famous symbols.
Later on, possibly fearing prying eyes or no longer liking what she had written, she covered them over with brown paper with an adhesive backing like a postage stamp, and their content remained a tantalizing mystery for decades. In 2016, conservators took photos of the condition of the diary during a routine check. Later, researchers realized the underlying text was partly visible and modern software could probably decipher it.
Frank apparently began an entry on September 28, 1942, then ruined the pages.
Now, through digital technology, researchers from the Anne Frank House museum and two other Dutch organizations have found that Frank hid four "dirty" jokes about sex and her perspective on sexual development, contraception, sex and prostitution.
The story is set in 1942 when a 13-year-old Jewish girl and her family fled their Amsterdam home and went into hiding following the Nazis' occupation of Holland. It is unclear when the text might be available in English, the Times reports.
But it also said that the pages "do not alter our image of Anne".
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"She starts with an imaginary person whom she is telling about sex, so she creates a kind of literary environment to write about a subject she's maybe not comfortable with", Bruijn told the Times.
Ronald Leopold, executive director of the Anne Frank House, said of the hidden pages, "Anne Frank writes about sexuality in a disarming way".
Frank's candid words on sex didn't make it into the first published diary, which appeared in English in 1952.
Anne died at Bergen-Belsen in Germany in early 1945, aged 15, less than a year after her capture and just before the end of the war. Then, in 1995, an even less censored version, including a passage on Frank's own body previously withheld by her father, was published.
But exactly when and exactly why Anne blocked out the pages will likely never be known. The inclusion of the pages in a biography of Frank sparked acopyright furor, and they were only released in a new critical edition of the book in 2001. "They make it clear that Anne, with all her gifts, was above all an ordinary girl", he added, according to the Associated Press.
"Anyone who reads the passages that have now been discovered will be unable to suppress a smile", he said.