Draw a single loop through the centres of some cells according to the following rules:
1. The loop may only travel horizontally or vertically, never diagonally; all turns are right-angled turns. The loop may only turn at the centers of grid cells. The loop may not cross itself or branch off; that is, from any cell centre, there can be only 0 or 2 loop segments extending from it.
2. The loop must pass through every circle.
3. The loop may not turn at a white circle, but must turn at at least one of the adjacent cells in the loop.
4. The loop must turn at a black circle, but may not turn at either of the adjacent cells in the loop.
Below is an example and its answer.