You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: WORD ORIGINS 
Clue: Like the name of a minor Roman god, this word for a caretaker comes from the Latin for "door"
janitor

Category: GEORGE WASHINGTON 
Clue: In 1798 George wrote to John Greenwood, a man in this profession, "I am...ready to pay what ever you may charge me"
dentist

Category: SCIENCE HISTORY 
Clue: This concept dates to a 1783 paper by John Michell, who theorized about a body with the sun's density & 500 times its diameter

a black hole