I really like this one. The solution is easy and requires only basic math and good pattern recognition. If you don't spot the pattern, however, you've got a slog ahead of you.
There are hundreds of squares in this picture with dimensions ranging from 1x1 to 12x12. Exactly how many squares are there?
I've got a more detailed pedagogical discussion of the problem at You Do the Math. I'll post the answer in the comments section of this post later in the week.