Draw a non-intersecting loop through the centers of some cells that passes through every circle and moves orthogonally between cells (ie never diagonally). The loop must turn on black circles and travel straight through the cells on either side. The loop must go straight through white circles, and turn in at least one of the cells on either side. All orthogonally connected groups of non-loop cells must be connected to the edge of the grid. For the solution check function to work, enter a 1 into every non-loop cell and 2 into every cell in which the loop turns.