obvious single
level 2 - easyalso known as: naked single, only one option
what it means
when you write down all possible numbers for a cell, if only one number is possible, that's the answer. it's the most straightforward technique in sudoku!
example 1: the lonely candidate
look at this cell. after checking what numbers already appear in its row, column, and 3×3 box, only one number can possibly go there.
the highlighted cell can only be 6
example 2: from many to one
sometimes you start with multiple candidates. as you fill in other cells, candidates get eliminated one by one until only the single remains.
before: multiple candidates noted
after: only 4 remains - obvious single!
example 3: multiple singles
a puzzle can have several obvious singles at once. each one you find makes the next one easier to spot!
multiple obvious singles can exist in a puzzle
🍦 the ice cream shop analogy
imagine you walk into an ice cream shop. you look at the menu and see:
- ✗ chocolate — sold out
- ✗ strawberry — sold out
- ✗ mint — sold out
- ✓ vanilla — available!
if only vanilla is left, you get vanilla! 🎉
that's exactly how an obvious single works — when only one number is possible, that's your answer!
how to find obvious singles
- pick an empty cell
- check which numbers (1-9) are already used in:
- the same row
- the same column
- the same 3×3 box
- write down (as small numbers) which digits are still possible
- if only one digit remains — congratulations, you found an obvious single!
quick summary
obvious single: a cell with exactly one valid candidate. when all other numbers are eliminated by row, column, and box constraints, the remaining number must be correct.