The Eight Queens Puzzle
The rules are simple. You must place eight queens on a chessboard in such a manner that no queen is on either the same rank, file, or diagonal as another queen.In the terminology of cognitive psychologists, this is a very knowledge lean, well defined problem.
What makes it difficult is the size of the problem space. There are 64 possible places to place eight queens, a total of 4,426,165,368 possibilities.