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 consistency, answers should be in all lower cased letters.

Category: DOUBLE DOUBLE LETTERS 
Clue: A gathering at which Pete Seeger & Joan Baez, or an owl & a goat might sing
hootenanny

Category: FAMILIAR PHRASES 
Clue: Pool table piece you don't want to be "behind"
the 8 ball

Category: WELCOME "BACK" 
Clue: In car-speak, it's a loud, premature explosion in the intake manifold

a backfire