Draw one closed loop through every pearl. It never crosses or touches itself, and it may skip empty squares.
At a black pearl the loop turns, then runs straight through the next square on both sides. At a white pearl it runs straight, and turns in the square before or after.
Tap between two squares — draws or erases that segment.
Drag across squares — draws along the way. Dragging back erases.
A dashed square marks a branch or a pearl whose rule is broken.
More controls
Hint shows the next forced segment, or one that has to go, and says why. Tap it again to apply.
Arrows move. Shift with an arrow draws or erases in that direction. Enter or Space puts the pen down or lifts it; with the pen down, arrows draw. Z or Backspace undoes. H hints. Restart clears this puzzle; Undo restores it.