For 20 years, one book for kids has been among the most frequently banned in the U.S.

Banning books is nothing to celebrate, except when it's Banned Books Week. During the last full week of September, the American Library Association sponsors events around the U.S. to raise awareness of books that have been challenged and stress the need for readers to be able to make their own decisions about their reading materials.

The top three reasons for banning a book are violence, curse words, and sexual content, NPR reports. And one book has the honor of being challenged almost since the first day it appeared on shelves in 1994: It's Perfectly Normal.

It's Perfectly Normal, meant for children 10 and older, discusses emotional and sexual health, relationships, puberty, and more, all while featuring photos of nude people. Author Robie Harris told NPR that she knew the book might raise some eyebrows and was warned by several people not to write it, "but I really didn't care. To me it wasn't controversial. It's what every child has a right to know."

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It's Perfectly Normal is now in its fourth edition, having sold more than one million copies. For each new edition, Harris has experts fact-check the book to ensure that updates on topics like AIDS prevention tips are correct. Issues that wouldn't have crossed anyone's mind in 1994, like sexting and internet safety, are new topics in the latest edition.

Some of the parents who have called for restricted access to the book say they don't want to see it banned, per se, just hidden from children in a restricted section of the library. One parent told NPR he doesn't want his children seeing the illustrations without him — which, essentially, would block the book from kids because who wants to look at naked drawings with their dad? Predictably, the thought of having It's Perfectly Normal in a designated area doesn't sit well with Harris. "No child's going to go up to a librarian and say, 'You know, I'm going through puberty, I'm having these changes... could you recommend something to me?'" she said. "If a book is in a special section of the library, maybe the kids who need it the most are not going to get it." --Catherine Garcia