Why gravity?

I think of a sudoku grid as a shelf of number blocks. Each block stays in its slot because something holds it up: the floor of the shelf, a block underneath, or one of the dark blocks that came with the puzzle. On paper, a number you write stays put forever, even a wild guess. When we built this game, we wanted a board where every number had a little weight to it.

So every minute or so, the shelf tips. Any number you placed that isn’t resting on something slides down its column until it lands. The way to stop that is to finish a whole row, column or box correctly. The numbers in it turn green and lock, and they never slide again. It turns sudoku into a gentle race to finish lines.

A bit of real sudoku history

The puzzle is younger than most people guess. Its modern form was most likely designed by Howard Garns, an architect in Indianapolis. Dell Pencil Puzzles and Word Games printed it in May 1979 under the name Number Place. Garns never got a byline. Puzzle editor Will Shortz later noticed his name in the contributor list of every issue that ran Number Place. Garns died in 1989, years before the puzzle became famous.

In April 1984 the Japanese puzzle company Nikoli ran it in its magazine under a long name, Suji wa dokushin ni kagiru, roughly “the digits must be single.” That got shortened to sudoku: su for number, doku for single. Nikoli’s founder, Maki Kaji, came to be called the godfather of sudoku.

It reached the rest of the world through Wayne Gould, a New Zealander who had been a judge in Hong Kong. He found a sudoku book in a Tokyo bookshop in 1997 and spent about six years writing a computer program to make the puzzles. The Times in London started printing them in November 2004. In 2012 a team led by Gary McGuire at University College Dublin used a supercomputer to show that a 9×9 sudoku needs at least 17 starting numbers to have only one answer. Our Big boards start you with 36 to 40, and every puzzle here has exactly one answer.

The four kinds of tiles

• Dark tiles are the given numbers. They never move.
• White tiles are the numbers you place. They slide when the board tilts.
• Green tiles are locked. A correct, complete row, column or box turns green and stays put.
• Soft red tiles mean a number appears twice in one row, column or box.

How to play

• Pick a size: Little (4×4, numbers 1 to 4), Middle (6×6, 1 to 6) or Big (the classic 9×9, 1 to 9).
• Fill every row, column and box with each number once. Tap a square, then a number on the pad, or use the arrow keys, number keys and Backspace.
• The bar counts down to the next tilt: every 60 seconds on Little and Middle, 90 on Big. The board wobbles for the last 5 seconds.
• When it tilts, loose numbers slide straight down until they hit the bottom or another tile.
• Calm mode on the start card turns tilting off.
• Puzzles solved and best times save on this device for each size.
• Sound is on to start. Tap 🔊 to mute, or 🎵 for music.

Tips

Build from the bottom. A number on the bottom row, or sitting on a dark or green tile, has nowhere to fall. Fill those spots first and the tilt can’t touch them.

Finish lines, not patches. Eight numbers scattered around the board can all slide. Eight that complete one row lock for good.

Use the wobble. When the board starts to wobble, drop in the number that completes a line. Save risky guesses for just after a tilt.

Red means look again. A line with a wrong number in it can never lock, so fix red tiles early.

Your turn

The board is at the top of this page. Start on Little if sudoku is new in your house, or go straight to Big and count the tilts you ride out. Pick a size and play.