Fatto generale: 15 is the number of balls in the eight ball variant of billiards.
Fatto matematico: 15 is a triangular number, a hexagonal number, a pentatope number and the 4th Bell number.
Trivia: 15 is the length in miles of a train of boxcars that can be filled from recycled paper in the USA every day.
Data storica: January 15th is the day in 69 that Otho seizes power in Rome, proclaiming himself Emperor of Rome, but rules for only three months before committing suicide.