Sudoku

The mini is 6×6 with the numbers 1 to 6; the classic is 9×9. Every row, column and box gets each number once. A wrong number is flagged straight away, and you can take as long as you like.

Guessing is free here, and that is exactly the trap: the number that feels seventy per cent right will unpick the whole board later. Scan for the number that already appears most on the board and place it everywhere it can only go, then move to the next. If nothing is certain anywhere, the 6×6 is the better place to build the habit than the 9×9.