Sudoku is a number puzzle. You get a big square cut into nine small blocks, and each block has nine little squares — 81 squares in all. Some squares already have numbers in them. Your job is to fill in the rest, so that every row across, every column down, and every one of the nine blocks holds the numbers 1 to 9, each one time only. You never need to add or count anything; you only work out where each number can and cannot go, so it is a thinking puzzle, not a maths puzzle — though a lot of people believe the opposite because they see numbers. In fact you could play it with nine colours or nine animal pictures and nothing would change. The word is a countable noun for one puzzle ("I did a sudoku on the train") and also an uncountable name for the whole game ("I like sudoku"). Many people also treat it as a plain noun with no article: "Do you play sudoku?" You see it in newspapers, in cheap puzzle books, and in phone apps, usually marked easy, medium or hard. English speakers say it roughly "soo-DOH-koo". It is not usually written with a capital letter, though older newspapers sometimes did.