Nobody makes jokes in Base 13
It was later pointed out that 6 × 9 = 42 if the calculations are performed in base 13, not base 10. Douglas Adams was not aware of this at the time, and has since been quoted as saying that "nobody writes jokes in base 13." and also "I may be a pretty sad person, but I don't make jokes in base 13."
http://en.wikipedia.org/wiki/The_Answer_to_Life,_the_Universe,_and_Everything