It is Halloween night and you have the good chair. A candle burns on the desk, rain drums outside, and every so often lightning throws the whole room white. In front of you: a grid with numbers along its edges. Fill in the right squares and a shape shows through, like frost wiped off a picture. If you know these as picture cross or griddlers, you already know the dance.

Every grid you crack hands you a piece of a bigger picture. Set it on the paper, turn it until it fits, press it home. A gentle tutorial teaches the rules from nothing. No timers, no lives, nothing to lose but the evening, and your progress is saved wherever you stop, so play for five minutes or stay until the candle is out.