Here is another version of that same riddle. Fascinating how the same thing gets told just slightly differently and it changes the joke quite a bit.
"What is it that hangs on the wall, is green, wet -- and whistles?" I knit my brow and thought and thought, and in final perplexity gave up. "A herring," said my father. "A herring," I echoed. "A herring doesn't hang on the wall!" "So hang it there." "But a herring isn't green!" I protested. "Paint it." "But a herring isn't wet." "If it's just painted it's still wet." "But -- " I sputtered, summoning all my outrage, "-- a herring doesn't whist…