Imagine opening the weekend paper and looking through the puzzle pages for the Sudoku. You spend your morning working through this logic puzzle, only to realise by the last few squares there’s no consistent way to finish it.
“I must have made a mistake,” you think. So you try again, this time starting from the corner you couldn’t finish and working back the other way. But the same thing happens again. You’re down to the last few squares and find there is no consistent solution.
