Draw a single continuous loop in the grid using horizontal and vertical line segments so that the loop passes every black dot, other cells may remain unvisited. It should not cross or overlap itself. Each dot marks the middle of the straight line segment it lies on.

Example puzzle

Grid control: To draw a line press the mouse button at a start cell and move mouse to another cell holding the button. You can click at the cell edge to mark it with a cross. More details about grid control are at the help page.