Sudoku has enjoyed massive and sustained popularity
Sudoku has furrowed the brows of a generation of commuters, but will it be replaced by a new puzzle from Japan?
"Easy" taunts the wording above the puzzle on the page. Yet this is anything but.
Now, there are plenty of people out there who wouldn't baulk at the challenge of a "Fiendish" sudoku or a "Deadly" Killer, so why is a number teaser billed as being so straightforward able to cause so much grief?
Meet kenken, the attitude-packed young cousin of the serene sudoku and the new kid on the number puzzle block.
All digits from one to six must appear in every column and row
Each block is demarcated by a thick line
The target number in the left hand corner is worked out using the other digits in the block using the calculation indicated by a symbol
Already a hit in Japan, with more than a million puzzle books sold, it has arrived in the UK complete with a fanfare from the Times newspaper, which is publishing the brainteasers.
The paper is billing kenken as "the new sudoku" - bold, considering the phenomenal success of the original, which took off in the UK four years ago, boosting newspaper sales and spawning thousands of books.
Since then variations on the theme have come and gone. Some - like the kakuro - have survived and have their own sets of devotees, yet none has challenged sudoku's supremacy.
But now a kenken puzzle has ousted one of the two sudokus on the back of the Times's features supplement.
Like sudoku, the smaller kenken consists of a numbers square where the figures cannot be duplicated within rows and columns.
But with the new puzzle, there's the added dimension of having to reach certain target numbers inside smaller blocks by adding, subtracting, multiplying or dividing the numerals in the cells within.
Japan is the source for the 21st Century's number puzzles
This makes solving a kenken a mind-stretching process of constantly evaluating competing theories and the teaser a strong contender for "puzzle most likely to make you miss your bus stop".
For creator Tetsuya Miyamoto solving a kenken involves the most basic learning instincts, and he says the trial and error process can even help the mind cope better with real-life problems.
The Japanese maths teacher claims each puzzle features "a trick, a discovery, a story" and is a way of stretching young brains, which he believes are failed by the rote learning involved in conventional teaching methods.
Chris Maslanka, "college enigmatist" at St Catherine's College, Oxford, and a teaser-setter for the Guardian newspaper, does believe puzzles like kenken can benefit the mind's flexibility and improve numeracy, but their power should not be overstated.
"This puzzle is individual. It's you pitted against the problem. One aspect of it is that it mirrors a primal need for figuring things out for yourself.
"Many people have a ton of problems. Real-life problems have loose ends that you can solve up to a point, but not always very satisfactorily.
"Here you have the satisfaction of doing what the brain is built for. It's a sort of inoculation against real-life problems.
"There is no quick fix, no royal road. You have to figure out your own strategy.
"That's a very helpful skill. What I like is that it develops what I call autonomous thinking. In a way these puzzles give you back your own autonomy on how to solve a problem."
Kenken translates as "cleverness squared" or "square wisdom", yet for Maslanka it is limited in that it is still "thinking inside the box", as opposed to outside it.
And while he prefers sudoku to kenken because it is more "aesthetic" and "minimalist", he admits he always feels like he's "dropping out" when taking on a puzzle.
"These puzzles don't have the richness of a crossword, which involves wordplay and humour. Sudoku is rather humourless. It is not a perfect puzzle - there is no such thing."
So do these apparent chinks in sudoku's armour mean that its number will soon be up - and is kenken the natural successor?
No, says Mr Maslanka, sudoku will be with us for some time to come. The arrival of kenken simply signals that we are heading for greater diversity in the range of puzzles being offered by our newspapers and publishers.
"Kenken is sudoku plus arithmetic. I don't think kenken is a replacement for sudoku, it's an addition, and people who want some more arithmetical meat will prefer it.
"The path we are heading down is that we will be individually served with the puzzles we prefer. In the future it will be horses for courses.
"It's a bit like spawning a lot of lifeforms and seeing which ones take. It's an experiment. And we will vote with our pencils."
Pencil - of course. Then you can always rub this one out and start again.