(CNN) -- Mystery has swirled around the two pages of Anne Frank's diary where brown paper was pasted over the writing. But today it was revealed what lied behind the covered up pages.
Thanks to modern image processing technology, the writing from 1942 was uncovered, and four "dirty jokes" appeared.
"I'll use this spoiled page to write down 'dirty' jokes," Anne wrote on September 28, 1942, just two months after she went into hiding.
She was 13 when she went into hiding from the Nazis with her family. She kept a diary throughout her time in hiding, and after her death, her father, Otto, published it in 1947.
In 2016, the diary was rephotographed by the Anne Frank House and the hidden writing was deciphered.
"Anyone who reads the passages that have now been discovered will be unable to suppress a smile. The 'dirty' jokes are classics among growing children," said Frank van Vree, director of the Netherlands' Institute for War, Holocaust and Genocide Studies. "They make it clear that Anne, with all her gifts, was above all also an ordinary girl."
What was on the pages?
Five crossed-out phrases, four dirty jokes and 33 lines about sex education and prostitution were included on the two pages of her first diary, according to the Anne Frank House.
One of her jokes read: "Do you know why the German girls of the armed forces are in the Netherlands? As a mattress for the soldiers."
Another one: "A man comes home at night and notices that another man shared the bed with his wife that evening. He searches the whole house, and finally also looks in the bedroom closet. There is a totally naked man, and when that one man asked what the other was doing there, the man in the closet answered: 'You can believe it or not but I am waiting for the tram.' "
A third joke: "A man had a very ugly wife and he did not want a relationship with her. One evening, he came home and he saw his friend lying in bed with his wife and the man said: 'He does and I have to!!!!' "
Finally, Anne Frank related this joke: "A man and a woman had a relationship, and after a few months the woman's belly was getting disturbingly big. Then, the man called a doctor who said: 'It's just air, Mrs., just air!!!" The man replied: 'I am not pumping air, am I?' "
On sex, Anne Frank writes about how a young woman will start getting her period when she is about 14, "which is the sign that she is ready to get into a relationship with a man, but of course one does not do that before one is married."
She goes with a candid description of sexual intercourse and contraception, before writing about prostitution: "All men, when they are normal, go with women, on the street such women talk to them and then they go together. In Paris, there are big houses for that. Dad has been there."
It's unclear why she decided to cover up these specific pages, but she mentions throughout her writings she was worried about people reading her private thoughts. She often went back through and added comments to previous journal entries.
"Anne Frank writes about sexuality in a disarming way. Like every adolescent she is curious about this subject. She also writes about it on other, uncovered pages," said Ronald Leopold, executive director of the Anne Frank House. "They bring us even closer to the girl and the writer Anne Frank."
CNN's Roseanne Roobeek contributed to this report.
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