Draw a single loop that connects the grid cells. The loop may only
travel horizontally or vertically, and never diagonally (so all turns
are of 90 degrees). The loop may only turn at the centers of grid
cells, and may not cross itself or branch off. Every grid cell must
be used, and the loop must enter and exit every region of the grid
exactly twice.
This image lacks the color and dotted lines of my
previous two entries due to time constraints. Sorry.
Edit: For solvers who insist on having color and dotted lines, I have found time to make
this image. The plain version above will still remain the default, though.