Color each square of a 6x6 checkerboard either black and white so that each square is a neighbor of an odd number of black squares. Here "neighbor" includes squares that touch only on a corner. A square is not considered to be a neighbor of itself.Variation for enthusiasts: What if 6 is replaced by n?

Source:1995 Quebec Math Contest, Crux Math. Nov 1997.© Copyright 1998 Stan Wagon. Reproduced with permission.

2 October 1998