Two Christmases ago, I received a book of Sudoku puzzles as a 'Secret Santa' gift (along with a V for Vendetta poster, but that's not important). I finished the 'Easy' and 'Mild' sections very quickly, but came to some struggles with most of the puzzles in the 'Difficult' section. I grew into the habit of doing as many numbers as I could, then giving up and moving onto the next one.
Recently I rediscovered the book of puzzles, and decided to give a go at finishing all the ones I couldn't do last year. To my surprise, I found myself able to complete all the 'half-done's easily. It felt nice to flip through the pages and see all the filled boxes. Knowing that I had become smarter, at least in this sense, I started to feel contentedly and smugly intelligent.
Then I realized I was measuring my intellect with Sudoku puzzles. The feeling went away.