This is a much loved computer science puzzle: you have a chessboard and eight queen pieces to place on it. The only requirement is that none of the queens threatens any of the others; that is, you must place them so that no two queens can capture each other.
This is a typical backtracking problem: you try one position for the first queen (in the first row), advance to the second, and so on. If you find that you are unable to place a queen, you backtrack to the previous one and try another position. Finally, you either exhaust all possibilities or find a solution.
queens()