Today's number: 15

Today is 15 December - the date of the last of these shameless plugs for The Book of Numbers by William Hartston (Richard Cohen Books, price pounds 9.99).

Fifteen is the number of:

babies in the largest recorded litter of a ferret;

people per square kilometre in Finland;

checkouts a supermarket has to have before it can call itself a hypermarket;

minutes everyone will be famous, according to Andy Warhol;

pounds of hydrogen in the average human body;

red balls in snooker;

men on a dead man's chest;

blows the executioner took to sever the head of Mary, Queen of Scots;

escalators at Baker Street tube station - the highest number on the London underground;

letters O in an Italian Scrabble set.

Fifteen is also - according to recent surveys - the percentage of women who would consider having sex after a first date with someone they have just met.