Su Doku, the Japanese puzzle game that is sweeping the nation, is also taking the classroom by storm. Schools across the country are flashing up the grids on whiteboards and laying them out on playgrounds.
Chris Harrison, headteacher of Oulton Broad primary in Suffolk, is challenging his junior pupils to complete the puzzles in just 15 minutes. A 3ft square grid goes up outside the Year 4 classroom each morning and is filled in by the end of breaktime.
"We put the given numbers on it in red and then challenge the children to complete the grid at playtime," he said.
"It started off with me photocopying grids for a few children and then interest grew," he said. He has now ordered a giant grid for the playground. Gill Dillon, a maths teacher at Abraham Guest high school in Wigan, found Su Doku has enthralled pupils at the specialist sports college.
She said: "The first time we did it, I started them off on the whiteboard and we filled some of it in together. Then I gave them 10 minutes to finish it off. But they wanted to carry on. Now, children are cutting them out of newspapers and bringing them in."
Mrs Dillon initially used the grids in a Year 8 class to introduce problem solving. She said: "We started using them about a month ago. It seemed a good way to explain logic, how to eliminate things and put things back."
Now she uses Su Doku as a "time filler" for students who finish work early.
She said: "Some children are asking for grids to take home and some are pulling them off websites."
At Oaks Park high school in Ilford, Stephen Froggatt, head of maths, uses the puzzle as a warm-up activity, projecting the grids on to a whiteboard so pupils can work together on them.
He said: "I want them to catch the excitement - we are all gripped by the puzzles.
"My aim is to encourage the children to do them and then tackle the ones in the papers. Solving a puzzle at the start of the lesson is one way in which we can get the youngsters thinking clearly.
"Su Doku is all about proof. You can't make guesses at it, and this is a gentle introduction to proof."
Mr Froggatt uses a computer program to compile the puzzles. As well as using the traditional grid, the school provides smaller ones for less able pupils.
Millions of adults have become addicted to Su Doku in a craze similar to the Rubik's Cube phenomenon of the Eighties.
To solve a puzzle, every row, column and three by three box within the grid needs to contain the digits one to nine. They can be cracked purely by logic: no mathematical skill is required.
Some schools use smaller grids, either four by four or six by six.
The puzzle was introduced to Britain by The Times in Novem-ber and the newspaper's puzzle book by Wayne Gould is now topping the bestseller lists.
In October, The Times National Su Doku championships, including a junior competition, will take place at the The Times Cheltenham Festival of Literature. More details will be published in The TES in the next few weeks.
PLAN PUZZLE LOGIC IN LESSONS
* Use Su Doku puzzles as a starting point to explain and discuss logic, leading onto a discussion about organisational skills.
* Project the grid on to a whiteboard enabling the class to do the puzzle together.
* Use smaller grids and easier puzzles with less able children so they can feel satisfied. Grid sizes and difficulty can be increased depending on the child.
* To encourage co-operation, make the children work in pairs or to encourage individual thought, make them work on their own.
* Have a Su Doku race.
* Carry a few puzzles around with you to hand out to children in wet breaktimes or lunchtimes.
* Attach a grid to the classroom door or a whiteboard or blackboard for children to do at breaktime or lunchtime. The given numbers on these grids can be changed every day, or you can leave a more difficult puzzle up for longer, erasing the incorrect additions after the children have gone home.
Solutions on page 16