Picross DS is another puzzle by numbers game originating from Japan (where it goes under the name of Oekaki Logic), in which you use the numbers written along the top and left-hand side of a grid as hints to fill in the squares on the puzzle and complete the hidden illustration.
The logic used to determine which squares in the grid to colour in, and which to leave blank (or marked with small x's) is the key to this game. To express the concept very simply, imagine a row of ten blank boxes. If you were told that eight adjoining (and that's the key here) boxes were to be coloured in, but you weren't told the starting box, you could still use logic to figure out which of those ten would be coloured in no matter what. Hint: it would be six of them.
There are two types of rules in Picross DS. During "Normal mode rules" you will be told if you fill in an incorrect square, while in "free mode rules" you will not be informed of your mistakes. (Easy mode and Normal mode use Normal Mode rules. Free mode uses free mode rules)
There is one level of easy mode, four levels of normal mode and four levels of free mode play. All games give you the option of viewing hints. The level of difficulty increases with the size of the grid, which starts at 5 x 5 and increases to 15 x 15. Free mode is the most difficult, and should only be used by intermediate to advanced players.