The infamous “Who gets it?” puzzle

(adapted from the rec.puzzles riddle I posted in June, 2001)

I don't get it; YOU get it.
SOUL doesn't get it; WIT gets it.
DOWN doesn't get it; UP gets it.
ON doesn't get it; OFF gets it.
TOE doesn't get it; HEAD gets it.

Does A SWAN or A DUCK get it?

Hint: (drag the mouse below this line to reveal)

What word can be put in front of each of YOU, WIT, UP, OFF, HEAD, but none of I, SOUL, DOWN, ON, TOE, to obtain a common [sic] colloquial expression?

Solution: (drag the mouse below this line to reveal)

A DUCK gets it (and rhymes with the key word).