The Herring Riddle

Saul and Abe are walking down the street...

Saul grabs Abe by the arm and says, 'Abe! I've got a riddle for you!' 'Okay, Saul. Go ahead.' 'What's green, hangs on a wall, and whistles?' 'I... You got me, Saul. What?' 'A herring.' 'But... herrings aren't green.' 'Well, you can paint it green.' 'And they don't hang on walls.' 'Sure! You put a little nail in it, you can hang it on a wall.' 'And... and it doesn't whistle.' 'Okay, so it doesn't whistle.'