Twenty-eight, being equal to the sum of the numbers that divide it exactly, (1+2+4+7+14) is a perfect number, the second such number, after 6. The next two are 496 and 8,128. It is still not known if there are any odd perfect numbers, though if there are, they have more than 200 digits.
Twenty-eight is also viewed as a perfect number from the mystical point of view. It is the number of days of the lunar cycle (or "mansions of the moon" as astrologers would have it) and also the number of letters of the Arabic alphabet, in which the Quran is written. That, according to the 11th-century mathematician, al-Biruni, was tantamount to a proof of the existence of God.
The link between the cosmos and the physical world is completed with the information that 28 is the number of days the human skin takes to regenerate itself completely.
Twenty-eight is also:
The number of countries that sold arms to both sides in the Iran-Iraq war.
The number of people per square km in the United States.
The pounds of carbon in an average human body.
The percentage of the world's tea grown in India.
The top speed of a lone lobster in centimetres per second.
More "26 L of the A" (Letters of the Alphabet) teasers.
2 W on a B
2 B in a M
2 L in a P of S
2 C on a T
2 F T I W for C
The first three correct answers opened on 10 October will win a Chambers Dictionary. Entries to: Pastimes, Independent, 1 Canada Square, Canary Wharf, London E14 5DL.
14 September answers: 5 More Alphanumeric Puzzles to Solve; 6 Pips in the Greenwich Time Signal; 12 Good Men and True; 19 Pence for a Second Class Stamp; 40 Whacks Lizzie Borden Gave Her Mother. Winners: Jean Carr, Michael Wallace, Peter Tutt.