Draw one single closed loop along the grid lines. A number says exactly how many of that square's four sides the loop uses.

• Click between two dots to draw a line, again for an × (definitely not a line), again to clear.
• A 0 means none of its four sides — mark them all with × first.
• A 3 next to another 3 forces the line between them.
• The loop never crosses or branches: every dot has 0 or 2 lines.

Slitherlink has been in Nikoli's magazines since 1989 and is the purest deduction puzzle here — every one of these has exactly one loop.