The applet below presents an old puzzle. The board is a 3x3 part of the chessboard.
The only allowed move is to drag a knight to another square following the usual chess rules.
Of course, no two knights may occupy the same square. The purpose of the puzzle is to swap
red and blue knights.
On the right, 8 squares are arranged in a sequence. There is correspondence between
the squares and knights on the two halves of the display. The sooner you grasp this relationship,
the easier it will be for you to solve the puzzle.