What's the next thing in this sequence?
1, 11, 21, 1211, 111221,
I didn't figure it out. I finally asked her about it on March 15, 2001, and she told me the next member: 312211.
I still didn't figure out the pattern.
On November 11, 2009, I discovered the On-Line Encyclopedia of Integer Sequences through hitting "Random" on Wikipedia. I remembered that old sequence that had stumped me, and plugged it in; it's A005150. (That link would certainly be a spoiler.)
Eleven years is probably not the longest gap between riddle and answer that I'll encounter. It may well happen that when I'm retired, I'll remember something from my teens and go "OOoohh, now I get it!"