Nurikabe is a logic puzzle about islands in a sea. Some squares hold numbers; your job is to shade the rest of the grid into one winding sea so that every number ends up in an island of exactly that size. Every puzzle here has exactly one answer, and you can always reach it by reasoning.
The rules
Each number belongs to an island: a group of unshaded squares joined side by side, with exactly as many squares as the number.
Every island has exactly one number. Islands may touch at a corner, but never along a side.
All shaded squares (the sea) must join into one connected piece.
No 2×2 block may be entirely shaded.
Playing on the board
Tap a square to shade it, tap again to mark it with a dot (island), and tap once more to clear it.
Drag across several squares to paint them all the same way. Dragging out of a number paints island dots.
Undo takes back your last change, and Clear wipes the board (it can be undone too).
The puzzle is solved as soon as exactly the right squares are shaded. Dots are notes for you.
Checking your work
Mistakes that are certain show up straight away: shaded 2×2 pools and cut-off patches of sea turn red, and so do islands with two numbers, too many squares, or not enough room. A number fades when its island is complete and walled in by sea.
Check rings any square you have marked wrong. Hint corrects a wrong mark first; otherwise it fills in a square that follows logically from the board.
Opening moves
Shade all around every 1: its island is already complete.
A square touching two different numbered islands, side by side or diagonally between them, must be sea.
Shade squares that no number can reach with the squares it has left.
When three squares of a 2×2 block are shaded, the fourth must be island.
A patch of sea with only one way out must continue through it to join the rest.
Difficulty levels
Easy is a 6×6 grid you can solve step by step with the basic rules. Normal is an 8×8 grid with bigger islands. Hard is a 10×10 grid where some steps need a quick "what if": imagine a square shaded, and if that breaks a rule, it must be island. Every puzzle is freshly made, and your best time is kept for each level.