42 is the number from which all meaning ("the meaning of life, the universe, and everything") could be derived.

42, the answer given by the supercomputer Deep Thought in The Hitchhiker's Guide to the Galaxy.

For seven and a half million years, this stupendously powerful, office-block of a machine had whirred. When it came to announcing what it had discovered, crowds had quite understandably gathered. "You aren't going to like it," Deep Thought warned. "Forty-two," it said, with infinite majesty and calm.

Why did Adams pick that number? Is there a connection to something the world doesn't know about? Is the CIA and the MI6 involved in all this? Real aliens, perhaps?

A BBC radio script based on Adams' book contains the following lines:

("Cave man" lays out following sentence in Scrabble stones: "What do you get if you multiply six by nine?")

Arthur: Six by nine? Forty-two? You know, I've always felt that there was something fundamentally wrong with the Universe.

(Faint and distant voice:) Base thirteen!

For the literal-minded and those unfamiliar with terms like "base thirteen," this is a number system in which the number 10 is equivalent to our more familiar decimal 13. A base-13 number 42, therefore, is the same as four 13s plus 2, or decimal 54. So "six by nine" (six times nine) or decimal 54 is, in base 13, 42. For the mystically inclined, 42 in base 13 is the same as 101010 in binary (base 2). This could mean almost anything, and many Adams fans have spent untold hours discovering all of the places where the number "42" pops up. For example, there are many mentions of the number in the Book of Revelation. Others have made a game of finding 42s, such as these:

- The angle at which light reflects off of water to create a rainbow is 42 degrees.
- Two physical constants in the universe are the speed of light and the diameter of a proton. It takes light 10 to the minus 42nd power seconds to cross the diameter of a proton.
- The sum of the ordinal alphabetic positions of the initials (SPG) for Stan (P.) Gibilisco, an oft-published science and technology writer, is equal to 42 (S=19, P=16, G=7).
- A barrel holds 42 gallons.

(It should be noted that all of these 42s are base 10, not base 13.)

The puzzle is an illustration consisting of 42 multi-coloured balls, in 7 columns and 6 rows. Douglas Adams has said,

“Everybody was looking for hidden meanings and puzzles and significances in what I had written (like 'is it significant that 6 * 9 is 42 in base 13?'. As if.) So I thought that just for a change I would actually construct a puzzle and see how many people solved it. Of course, nobody paid it any attention. I think that's terribly significant.”

In the puzzle the question is unknown, but the answer is already known to be 42.