Hecho general: 21399 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 21399 is an uninteresting number.
Trivia: 21399 is an unremarkable number.
Fecha histórica: August 2nd is the day in 1873 that the Clay Street Hill Railroad begins operating the first cable car in San Francisco's famous cable car system.