Yes, you've got it now! MEGABYTE, you may provide a riddle. If not, it will be bobman37's turn.
The pattern is to describe the sequence. With 11 and 21, you are saying first that there are "two ones," equaling 21. Then, it would be one two, one one (1211), and etc. I was taught this riddle, and I myself didn't figure it out. To tell the truth, I'm not that good with riddles, brain teasers, or any of the sort.