Sudoku (soo-doh-koo): a 9x9 logic puzzle to superheat the very core of your brain and to challenge your numeracy, spacial awareness and problem-solving abilities.

WARNING: the five levels of Penguin Sudoku Challenge: Volume 2 may leave you smugly grinning, scratching your head, sighing deeply, snapping your pencil - and you may even throw your book at the wall.

But you also have permission to frame your most difficult completed 9x9s.

Good luck!