I just emailed my brother BOB to ask him where we'd heard the riddle:
Q-What's the difference between a duck?
A-One leg is both the same.

He sent back this:

Q-What's the difference between a duck?
A-Jeez and quackers

I seem to recall Mom and Dad came home from a "couples' club" at Binkley Church, and someone (maybe Bob Cruikshanks) had told that joke, and it hit the spot with them so they repeated it numerous times (laughing or at least smiling every time) and, alas, it became part of our family folklore.
The eggplant grunch was definitely from Mad Magazine, and its incomprehensible logic appealed to you and me (and probably only you and me) in much the same way as "one is both the same" appealed to our parents. And I always remember a Walt Kelly Pogo cartoon that we cut out and had thumbtacked up in our room for years: It was just one selected square of a three square cartoon episode that contained two little bugs, a father and his young son holding his hand, the father enthusiastically pointing with his umbrella at Churchy LaFemme galloping by on all fours like a rabid dog, and the father bug saying, "Behold, Clerlock! There goes a..." and the bored child saying, "Never mind, Pa." It's hard to explain but that was quietly hilarious to us, and I think it still is.