I did not make this up:*
The second greatest computer of all time and space was built to tell the answer to the question of life, the universe and everything. After seven and a half million years the computer divulged the answer: 42.

"Forty-two! Is that all you've got to show for seven and a half million years' work?"

"I checked it very thoroughly," said the computer, "and that quite definitely is the answer. I think the problem, to be quite honest with you, is that you've never actually known what the question is."

The computer informs the researchers that it will build them a second and greater computer, incorporating living beings as part of its computational matrix, to tell them what the question is. The result is the sentence "WHAT DO YOU GET IF YOU MULTIPLY SIX BY NINE".

"Six by nine. Forty-two."

"That's it. That's all there is."

Since 6 x 9 = 54, this being the question would imply that the universe is bizarre and irrational. However, it was later pointed out that 6 x 9 = 42 if the calculations are performed in base 13, not base 10.

"42" is often used in the same vein as a metasyntactic variable; 42 is often used in testing programs as a common initializer for integer variables.

There is a joke that perhaps there may have been some order of operations issues:

Six equals 1 + 5.
Nine equals 8+1.
So six * nine equals 1+5 * 8+1.
5*8 = 40.
1+40+1 = 42, the meaning of life.

In Lewis Carroll's book The Hunting of the Snark, (before Douglas Adams' tome was written) the baker left 42 pieces of luggage on the pier.
42 is also a sphenic number, a Catalan number and is bracketed by twin primes.

42 is the number you get when you add up all the numbers on two six-sided dice. This is showing that life, the universe, and everything is nothing but a big game of craps.

According to Google's calculator, the meaning of life is, indeed, 42. Weird.

* This was all actually made up by Douglas Adams in his comic science fiction series The Hitchhiker's Guide to the Galaxy.
The information on this page was originally found at http://en2.wikipedia.org/wiki/The_Answer_to_Life,_the_Universe,_and_Everything.