By Michael Hartley
The "eight queens" puzzle is a well-known puzzle, going back to 1848. The problem is to try to place eight queens on a chessboard so that none of them are attacking any of the other - remember that the queen can move as far as she likes vertically, horizontally or diagonally. You can't place two queens in the same row, for example, so on a normal chessboard (8 by 8) you can't put nine or more queens. Eight is the maximum.
The puzzle below shows an empty 4 by 4 chessboard, all in gray. The 4 by 4 board is easier than the 8 by 8, so it's a good size to start with. Click on a square to place a queen, and the square will turn green. The squares that queen is attacking will turn blue. See if you can place four queens on the 4 by 4 board! To remove a queen, click on it again.
If you get stuck, you can tap the 'reset' button at the bottom of the board. You can also try more challenging puzzles on larger boards. Can you master the eight by eight board?
I've also made an Android version of this puzzle for you to try on your phone. The game play is slightly different - you have to solve the 4x4 grid to uinlock the 5x5, and then solve that to unlock the 6x6 grid, and so on.
Have fun!