Some really enjoy having their brains challenged by difficult puzzles. I've heard the popular reports of how such stimulation can help ward off memory problems and dementia in old age.
If you enjoy a good challenge, try out noodle twisters from Conceptis Puzzles.
I tried a couple and decided I'm just not made for these sorts of things. They make my eyes cross and I get sleepy. But, for people like my husband who likes to play Sudoku and other such puzzles, the Conceptis Puzzles are right up his alley. And, yes, they do come in a mobile version for those of us who need something to occupy us while in the Dr's office or DMV line or whatnot.