Fatto generale: 15 is the number of letters in the words "uncopyrightable", "dermatoglyphics", "misconjugatedly", and "hydropneumatics", the longest English words with no repeat letters.
Fatto matematico: 15 is the 4th discrete semiprime (3.5) and the first member of the (3.q) discrete semiprime family.
Trivia: 15 is the legal age for sexual intercourse in many countries.
Data storica: January 15th is the day in 1933 that a twelve-year-old girl experiences the first Marian apparition of Our Lady of Banneux in Banneux, Belgium.