Look at one empty square's row, column and box together. If they already hold eight different numbers, the ninth goes in the square.
123456789
123456789
5
6
7
8
1
2
3
4
9
Numbers it can see9The answer it gives
Naked or hidden
A naked single asks which numbers fit one square. A hidden single asks which squares fit one number. Both place a number; they look from opposite sides.
How to spot one
Look where a busy row crosses a busy column. The more numbers meet at a square, the fewer are left for it.
With pencil marks
If you keep pencil marks, a naked single is a square with one mark left in it.
Practise Naked single
Boards that need it, with the owl's hints when you're stuck.