Fatto generale: 40 is the percentage of U.S. paper currency in circulation that was counterfeit by the end of the Civil War.
Fatto matematico: 40 is an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number.
Trivia: 40 is the number of thieves in Ali Baba and the Forty Thieves and in Ali Shar and Zumurrud, from Thousand and One Nights.
Data storica: February 9th is the day in 1895 that William G. Morgan creates a game called Mintonette, which soon comes to be referred to as volleyball.