Writing about homosexuality is one way to end up on the most challenged books list.
Schools have come a long way from the days of banning students from reading "The Adventures of Huckleberry Finn," "The Call of the Wild" and "The Catcher in the Rye," but plenty other books are still being pulled from the shelves. Here are some of the most frequently challenged books of 2013.
"Captain Underpants and the Revolting Revenge of the Radioactive Robo Boxers" by Dav Pilkey
In this kids' book, Captain Underpants and his friends use their special Purple Potty to go back in time 65 million years to find out why dinosaurs became extinct. Complaints have been filed about the book being too vulgar for the targeted age group.
"Fifty Shades of Grey" by E.L. James
No surprise here. The racy, S&M-heavy scenes in the year's most talked-about book is not a favorite with school librarians.
"And Tango Makes Three" by Justin Richardson and Peter Parnell
Penguin Tango's family isn't like the other penguin families at the zoo: He has two dads. The book was written with the intention of showing children that kids with gay parents are just like any other family, but some schools don't think the message is appropriate.
"Looking for Alaska" by John Green
Though author John Green's novel "The Fault in Our Stars" is currently being made into a movie, schools have complained about his first book, "Looking For Alaska," about a tragedy that happens on a school trip, as having offensive language and being too sexually explicit.
"Thirteen Reasons Why" by Jay Asher
In this book, a high school boy discovers a cassette tape recorded by a girl at his school who had committed suicide. She lists 13 reasons for why she killed herself, and the boy is one of them. The book has been banned from schools for being unsuitable for the targeted age group, with offensive language and sexually explicit text.
"The Absolutely True Diary of a Part-Time Indian" by Sherman Alexie
The story of a young man leaving his Indian reservation to go to an all-white high school has been called racist and criticized for having offensive language.
"The Kite Runner" by Khaled Hosseini
Although "The Kite Runner" has become recommended reading in many colleges, several high schools and middle schools want it pulled from their shelves because of religious and homosexual themes.
"Beloved" by Toni Morrison
Now roughly 10 years old, Toni Morrison's book about slavery landed on the year's list of the most challenged books because the religious themes, violence and sex raised eyebrows.
"Scary Stories" by Alvin Schwartz
Alan Schwartz's spooky tales have been popular with kids since the '80s, but this year, many schools no longer think his stories are suitable for the intended age group.
"The Glass Castle" by Jennette Walls
This true story about growing up poor has been criticized for being too sexually explicit and containing offensive language.