I am trying to understand something that should be simple. I have a "easy" puzzle here, and immediately found where 8 should be in the middle box.
But in various Sudoku Apps, they show "candidates" to the user. And sometimes there is only one candidate available, essentially giving away an answer for free.
It says that 6 and 7 MUST be in those boxes, but I can't see why, it is not obvious at all to me.
Ideally I would rather disable 'candidates' and find such candidates myself. What strategy can be used to show, just by glancing, that 6 and 7 are the only possibilities in those boxes?