Douglas Adams
Author Douglas Adams at his Santa Barbara home in 2000 Dan Callister/Online USA/Getty

Today is Towel Day, an annual celebration of the science fiction author Douglas Adams and his work on 25 May.

It celebrates Adams' The Hitchhiker's Guide to the Galaxy series - which tells the story of Arthur Dent, a man who explores the universe in his dressing gown.

So why is it called Towel Day?

As described in Hitchhiker's Guide, a towel is "about the most massively useful thing an interstellar hitchhiker can have" - so diehard fans choose to remember Adams by carrying one.

Adams wrote: "A towel ... is about the most massively useful thing an interstellar hitch-hiker can have. Partly it has great practical value.

"You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon...

"You can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough."

Five things you need to know about Douglas Adams and his work

The date for Towel Day was chosen randomly. Two weeks after Adams died in 2001, fans organised an annual holiday on 25 May to celebrate his life and achievements.

Adams was a committed environmentalist and campaigned for animal rights. He once took part in a charity trek, which involved wearing a rhino suit while climbing Mount Kilimanjaro.

Adams was friends with biologist Richard Dawkins. In an obituary published in the Guardian, Dawkins wrote: "Science has lost a friend, literature has lost a luminary, the mountain gorilla and the black rhino have lost a gallant defender (he once climbed Kilimanjaro in a rhino suit to raise money to fight the cretinous trade in rhino horn), Apple Computer has lost its most eloquent apologist."

Fans have come up with various theories as to why Adams chose the number 42 as "the answer to the ultimate question of life, the universe and everything". Adams dismissed any theories over the significance of the number, saying: "The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base 13, Tibetan monks are all complete nonsense. I sat on my desk, stared in to the garden and thought 42 will do. I typed it out. End of story."

Adams described himself as a "radical atheist" - so people would not confuse his beliefs as agnostic. Dawkins dedicated his book, The God Delusion to Adams.