Wikipeida article on Sudoku, a logic game that seems to have come together over time, gotten turned into a game in the US, but become a raging sensation in Japan before spreading around the world recently.
It looks fun. You need to make sure that the numbers 1 through 9 occur only once in each row and column and once in each 3x3 grid. Apparantly it can be quite tricky to solve some puzzles. I've downloaded a sample and will give it a try.
This might be a fun thing to expose Janneke to, even if she can't complete one yet. I hope to be able to inspire my kids to be curious or to remain curious, rather.
The Wikipedia article gives a number of links to sites one can download puzzles from, including this one.