You put digits from 1 to 9 in the empty squares. Each column and row should have one and only one unique digits. In addition, each of the nine smaller 9×9 grids should only contain one of each digit. In other words, no duplicate digits in any columns, rows, or marked inner 9×9 grids.

If you like doing Sudoku puzzles, you will like this variant of the puzzle by Scientific American known as “Sci-doku”. It uses letters instead of numbers. As with normal sudoku, each row, column, and nine-cell grid must contain one and only one of the nine letters. You can not just use any nine letters, you have to use the letters that are already shown on the grid.

Here are the full rules.

Play the Sci-doku games »

Play the game at mah-jongg.ch/reversi/

This is the classic game of Othello where you play against a computer opponent. Consisting of black and white stones, you place your color stone on the board causing the opponents color stone to flip to your color. Whoever has the most colored stones at the end wins. What is interesting about this game is that it has unlimited undos.

Play the game at mah-jongg.ch.

This game was programmed by Thomas Weibel using DHTML and Javascript. The object of the game is to remove tiles that are “open” on the left and the right. Don’t worry, if you can’t see any open tiles, there is a “hint” button that may reveal moves that you did not see. There is also a “zoom” button if you find the tiles to be too small and is straining your eyes. Note that not all games can be won. However, good players can sometimes win one out of every three games.

Play the game at mah-jongg.ch/mastermind/.

Programmed by Thomas Weibel, this solitaire game of mastermind requires a bit of logic.

Do you Sudoku? Play sudoku online at mah-jongg.ch/sudoku/.

A logic game where you have to use your deductive skills. Fill in the grid of numbers such that any row, column, the designated 3×3 sub-grids contain all the digits from 1 to 9.