The game of Sudoku seems to have replaced the humble crossword as the king of bus travel pursuits.

What's more, the way people are playing the game is changing too, and more and more people are using their mobile devices to crunch numbers on the move. Impossible Sudoku bills itself as 'the most powerful mobile Sudoku game', and while I've not tested all of its rivals, it's certainly up there with the best of the ones I've tried.

When you first load Impossible Sudoku you'll probably fall out of your chair when you see the game board. Unlike the games you get in puzzle books this one has loads and loads of little numbers as well as bigger numbers filled in every square on the grid. It's a little daunting at first but don't worry because these are here to help you. They are actually 'auto marks' that indicate the numbers that are available to enter in a particular square based on the current state of the grid. Don't worry if it's too confusing for you because you can switch the feature off (you can't toggle auto marks, unfortunately and the change will come into effect in the next game).

There are lots of other things you can customize in Impossible Sudoku, and it's actually one of the most impressive number games I've seen in terms of its options. You can undo, redo, mark, check moves, solve the puzzle, and access all manner of statistics. There are five levels of difficulty, and you can even create your own puzzles from scratch.

The presentation of the game is pretty impressive - there's a choice of nine bright, colorful skins and the sound effects that play when you enter numbers are quite cool without being annoying.

In fact, the only thing that really lets the game down is its feeble help system. All you get is a one-paragraph description of the rules and there's nothing in the way of tips and tactics that might help beginners.

Overall though, this is a wonderful simulation of Sudoku for mobile devices.