If you are looking for a new challenge when you have finished your Sudoku or Kakuro Puzzle, there is now a new Japanese brain teaser for you to try; Nurikabe. Invented and named by Nikoli, Nurikabe is a binary determination puzzle - Cell Structure and Islands in the Stream are other names which it is known by. A "nurikabe" in Japanese folklore is an invisible wall that blocks roads and upon which delays in foot travel are blamed.
According to Wikipedia
"The puzzle is played on a grid, typically rectangular with no standard size. Some cells of the grid start containing numbers. The goal is to determine whether each of the cells of the grid is "black" or "white" (Islands in the Stream calls these "water" and "land" respectively). The black cells form "the nurikabe" (Islands in the Stream calls it "the stream"): they must all be orthogonally contiguous (form a single polyomino), number-free, and contain no 2x2 or larger solid rectangles (Islands in the Stream calls such illegal blocks "pools")."
"The white cells form "islands" (which is where Islands in the Stream got its name): each number n must be part of an n-omino composed only of white cells. All white cells must belong to exactly one island; islands must have exactly one numbered cell. Solvers will typically shade in cells they have deduced to be black and dot (non-numbered) cells deduced to be white."
If that seems a little complicated(!) Nikoli have produced an online tutorial which you can find here
If you want to learn how to play or indeed are already addicted to Nurikabe, Sudoku guru Sam Griffiths-Jones has been quick off the mark - The Nurikabe Book: An Introduction with 101 Puzzles is availabe at Amazon.co.uk and Amazon.com