All through September, avid readers in Washington hunt for books someone thinks they shouldn’t be allowed to read.

In local coffee shops, libraries and bookstores, participants are finding parcels wrapped in black paper: Inside are books that have been the targets of censors.

The #UncensoredDC scavenger hunt, organized by the District of Columbia Public Library, is one of a number of events throughout the U.S. around Banned Books Week, which celebrates free and open access to information.

“This is really a wonderful way for us to celebrate America’s freedom to read: Look at what somebody doesn’t want you to read,” said James LaRue of the American Library Association, which sponsors Banned Books Week.

The scavenger hunt raises awareness of book censorship and the rights afforded to Americans under the First Amendment of the U.S. Constitution, which guarantees freedom of expression and freedom of the press.

Attempts to ban books are rare in the U.S., and organizations like the American Library Association want to keep it that way.

In 2016, there were nearly 370 “challenges” in which someone tried to get a book removed from a school curriculum or from public library shelves. America has an estimated 90,000 public libraries or schools containing millions of books.

LaRue says librarians watch the number of challenges with care. “We’re like fire-spotters — you want to catch it when it’s small.”

Some of the books in Washington’s scavenger hunt are considered literary classics, such as Margaret Atwood’s The Handmaid’s Tale, Yevgeny Zamyatin’s We and Lois Lowry’s The Giver. A common reason given for challenging a book is that a book is inappropriate for a certain age group or grade in school.

The freedom to express views and consider other views is something Americans take pride in, LaRue says.

“There’s something quintessentially American about the idea that no matter what you believe, no matter what side of what issue you’re on, you can walk into a public library and you can find something about it,” he says.