This is the sort of conversation that I would often have with my friends as a child, and I'm relieved to disover that I still find them interesting now.
Earth is destroyed, radioactive ash. Years later, an alien race discovers a scrap of paper in space that has miraculously survived the planetary destruction. Assuming you were the human that wrote the note on the scrap moments before the global cataclysm, what would you like it to say? Assume that this is the one thing that the aliens will read about humanity, and also assume that they can translate whatver sentence you write regardless of the language (pictograms are fine too. )
I think this is the one I thought of when I was 8, and decided to reconstruct it by dangerously burning scraps of paper by candlelight and using my 8 year old Mustek scanner to capture it. I was torn between this:
"Time flies like an arrow; fruit flies like a banana."
simply because it took me almost two decades to finally understand it.