One from when I was a kid
I remembered after reading my little sister's journal. Now disclaimer - she's actually a bit of a freak genius.

She was jumping on her bed when my older sister pipes up:
"You should be careful or you'll split the atom." Hannah stops.
"What does that mean?"
"The atom is a really tiny thing that everything is made up of."
Feeling evil, I joined in at this point.
"If you split an atom there's this really big explosion and you'll die. You've got to walk lightly and not jump."
Hannah gets scared and ever so carefully goes down stairs to talk to mum. She comes back up a few minutes later.
"I know you're lying, mum told me you can't split the atom without a special machine."
Remembering an advertising campaign that was going at the time I replied, "Ah, but you can with a Nissan."
My mum still recants the tale of being begged by a distraught younger daughter not to drive a Nissan.
(Guineapiggy Fuzzeh!, Wed 12 Jan 2005, 2:53, Reply)