Masyu is popular puzzle game in Japan.
Masyu is played on a rectangular grid of squares, some of which contain circles; each circle is either "white" (empty) or "black" (filled). The goal is to draw a single continuous non-intersecting loop that properly passes through all circled cells. The loop must "enter" each cell it passes through from the center of one of its four sides and "exit" from a different side; all turns are therefore 90 degrees.
The two varieties of circle have differing requirements for how the loop must pass through them:
White circles must be traveled straight through, but the loop must turn in the previous and/or next cell in its path;
Black circles must be turned upon, but the loop must travel straight through the next and previous cells in its path.