Naked Single

A classic Sudoku contains 27 containers (nine rows, nine columns, and nine 3x3 blocks). Each container contains 9 squares, and must contain each of the numbers from 1 through 9.

It is often the case that a cell can only take one value, when the contents of the other cells in the same row, column and block are considered. This happens when, between them, the row, column and block use eight different digits, leaving only the remaining digit available for the cell.

For example, in the partial Sudoku puzzle below, the cell marked with the asterisk can only be a 6. All other digits are excluded by the contents of the containing row, column and block.

So why is this technique called naked single? Simply because if you mark all the possibilities, these cells stand out because they only have a single candidate each. Consider this puzzle (from my interactive collection), in which 7 naked singles are clearly visible (I used the “Set up marks” feature on the ? menu). I've hilighted two of them in pink. Can you spot the five others?