Fatto generale: 21 is the number of points required to win a game in badminton and table tennis (before 2001).
Fatto matematico: 21 is the smallest natural number that is not close to a power of 2, 2^n, where the range of closeness is +- n.
Trivia: 21 is the number of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915.
Data storica: January 21st is the day in 1968 that a B-52 bomber crashes near Thule Air Base, contaminating the area after its nuclear payload ruptures.