I used to teach English to young kids in Korea about ten years ago. The curriculum was set by the school, and a common task was doing word search puzzles. Some students were adept at finding words, and many other students would try to get through the task as quickly as possible by cheating off of them. Aside from the obvious strategy of rule enforcement, one way to discourage cheating is to simply give each student a different puzzle. The words to find are the same, but the arrangement of letters is different for each student.

It is trivial to write a program to generate word search puzzles, so I decided to make the problem more interesting by adding a difficulty parameter. I could generate puzzles with a range of difficulty by varying the following factors:

* direction -- more difficult puzzles have answers in diagonal and/or reverse directions

* density -- more difficult puzzles do not necessarily have the answers uniformly distributed in the grid but can have dense groupings

* dummy words -- since some students search for any words in the grid, you can fill it with words that are not actually target words to make the task more difficult

* decoys -- you can fill the grid with slight deviations of target words in order to make finding the actual target more difficult