The New York Times: "Ken-Ken Puzzles"

This activity is optional. Attempt to complete one of these puzzles. Note that you can choose the level of difficulty (easier, medium, and harder). After a few practices, challenge yourself to attempt a Ken-Ken puzzle that is at the next level of difficulty. Do not allow yourself to get addicted!

Click http://www.nytimes.com/ref/crosswords/kenken.html?_r=0 link to open resource.