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.