Sudoku

Goal

Fill every empty square with a number from 1 to 9. Each column, each row and each 3×3 box marked by the thick lines may hold a number only once.

Playing

Use the numbers that are there from the start to work out what belongs in the empty squares. Pick a square, then tap a number below the grid to enter it. The starting numbers cannot be changed. Before entering a number, check that the same row, column and 3×3 box do not already hold it. "Undo" takes back the last entry.

Tactics

Start where the grid is full

Look first for the rows, columns and 3×3 boxes with the fewest empty squares. Run through 1 to 9 against what is already there and list what is missing. If a single square is empty, the one missing number belongs in it. Once you write a number in, re-read the row and column that cross that square.

Narrow the candidates in one square

Pick an empty square and rule out every number already in its row, its column and its box. If one number is left, it belongs there. While several remain, do not guess — move to another square. How empty a square looks tells you nothing about what may go in it.

Find the home of one number

Next, take one box and find where a particular number can go. If the 5 is missing, use the fives in the other rows and columns to rule squares out. If one square survives, that is where the 5 goes. Every number you enter changes the picture, so go back to the squares you could not settle earlier.