Seven

Q. There's no magic in the number 7, but it does have an uncanny way of turning up places. Such as where?

A. Seven is a prime number, with no divisors except one and seven, says Richard Phillips in "Numbers: Facts, Figures & Fiction." More famously, there are the Seven Deadly Sins (avarice, envy, gluttony, lust, pride, sloth and anger), the Seven Wonders of the Ancient World, Shakespeare's Seven Ages of Man, the Seven Levels of Hell, the Seven Dwarves, and of course, seven days in a week. The 1960 movie "The Magnificent Seven" was based on the 1954 Japanese film "The Seven Samurai." The soft drink 7-Up was originally called Bib-label Lithiate Lemon-Lime Soda (gulp!) but didn't catch on until the name change.

And in 1956 George Miller wrote of "The Magical Number Seven Plus or Minus Two" to describe the limits of human memory: If someone gives you four random digits, such as 6 6 2 5, you will probably be able to recite them every time, likewise with five digits or six or seven, but beyond this people start making lots of mistakes. "The longest number of digits people get completely right is called their 'digit span' and for most people this is about seven digits."

Courtesy of Richmond.com