general_fact: 21993 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 21993 is an unremarkable number.
trivia: 21993 is a boring number.
historical_date: March 18th is the day in 1965 that Cosmonaut Aleksei Leonov, leaving his spacecraft Voskhod 2 for 12 minutes, becomes the first person to walk in space.