If you see someone carrying around a towel all day, don't panic.
May 25 is Towel Day, an annual tribute to the late Douglas Adams, author of The Hitchhiker's Guide to the Galaxy and four of its sequels. Fans celebrate the day by carrying a towel with them at all times, because, as the guide states, a towel "is about the most massively useful thing an interstellar hitchhiker can have."
The Hitchhiker's Guide is a science-fiction comedy that originally began as a radio series broadcast by the BBC in 1978. It was later adapted into a "trilogy" of six books, five which were written by Adams -- The Hitchhiker's Guide to the Galaxy, The Restaurant at the End of the Universe, Life, the Universe and Everything, So Long, and Thanks for All the Fish and Mostly Harmless. A sixth book, And Another Thing..., was written by Eoin Colfer in 2009.
The series has also been adapted into stage shows, comic books, TV series, computer games and a 2005 movie starring Martin Freeman and Zooey Deschanel.
Towel Day was first celebrated in 2001, two weeks after Adams' death and fans are encouraged to tweet, blog and post photos of their towels.