I've been pondering Sudoku recently. It is easy [I think] to generate a sudoku grid. But beyond that there are a bunch of questions about turning it into a puzzle that I haven't seen much talked about. Most of the articles I've found fret about numbers, rather than algorithms [minimum number of clues necessary, how many grids are there with the, the maximum number of clues and still have an ambiguous grid, etc] I'm wondering about such things like: after you generate a grid, how do you figure out which cells to give as clues, recalling that the clues should be optimal for that grid [that is, yield a unique solution and have the solution not be unique if any clue is removed]? How do they determine the "difficulty" of a puzzle? /Bernie\ Bernie Cosell bernie@fantasyfarm.com -- Too many people; too few sheep --