Rating the difficulty of puzzles

MindYourDecisions challenge problems range from fairly easy to near impossible. We must find a way to rate problems that's useful to students, maybe challenging, very challenging, and extremely challenging. What follows is tentative. Please contact me if you have a better idea.

Challenging: You should attempt this problem and give it significant effort. You are likely to find the solution given enough effort.

Very challenging: May require a clever construction or application of a theorem in a way you've never seen.

Extremely challenging: The solution will require grueling effort. I don't think these are worth indexing. Maybe I will index them later if I find some students can actually solve them. I don't want to give people such hard problems that they only get discouraged when failing to solve them. Here are problems which I would rate extremely challenging.