The basicsBasic boards
Hidden single
When a number fits in only one square of a row, column or box, it goes there, even if that square could hold other numbers too.
The box being searched7The answer it gives
Why it hides
The square itself could still take two numbers, so looking at it alone shows nothing. The answer appears when you look at the box.
How to spot one
Take one number and go box by box. Where a box is missing it, extend the lines of every copy nearby and see what's left.
Rows and columns too
The same search works along a row or a column: if a number has one square left in a line, it goes there.
Practise Hidden single
Boards that need it, with the owl's hints when you're stuck.