I've been compiling a list of things I excitedly told my kid brother 20 years ago, which he (annoyingly) scoffed at.  These were ideas (from any number of sources) that captured my imagination but which irritated my brother's skeptical brain and stimulated his argumentative nature:

1. Had dinosaurs not died out, they would have evolved into human beings.  [I had seen a computer model proving this one, with an illustration of what a dino-human would have looked like (scaly skin, lizard-like features, human frame).  In fairness to me, this was long, long before the general public had any reason to doubt computer models.  So-called evidence aside, I'd say my brother's suspicions about this one were overly exaggerated.  Here's a short article about the theoretical "dinosauroid."]

2. The Navy cannot train dolphins to plant underwater bombs, because dolphins are pacifists.  [I still like the idea of dolphins being pacifists.  I heard this one from my professor of transformational/generative grammar.  He didn't have the Navy's unclassified reports on hand.  Here's a brief mention of "the dolphin who refused to fight" in the Persian Gulf.]

3. Eskimos have hundreds of words for "snow," proving that different cultures experience different realities.  [This is indeed an urban legend.  My brother was right, though not necessarily for the right reasons.  Here's a Wikipedia article about the origins and significance of the myth.]

4. The only reason dolphins don't paint, sculpt, play instruments, and build buildings is that they don't have hands.  [In other words, dolphins don't have a culture due to a physical handicap, not because they're otherwise unevolved.  I still like this idea.  Here's an article entitled "Dolphins and Man — Equals?"]

Well, that's all I can think of right off the bat, though I recall my brother scoffing at me hundreds (if not thousands) of times.  Luckily, kid brothers don't carry much clout ... though here I am 20 years later still thinking about what mine scoffed at!