general_fact: 9991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 9991 is an unremarkable number.
trivia: 9991 is an uninteresting number.
historical_date: May 9th is the day in 1961 that Jim Gentile of the Baltimore Orioles becomes the first player in baseball history to hit grand slams in consecutive innings.