A demonstration of the irrationality of jigsaw puzzles: one endeavours to assemble... a picture one already has on the front of the box. (‘That's a pretty picture!’ – Yes, but why should that matter when assembling a replica as an intellectual pursuit?)
Now some of these are chosen on grounds of difficulty: the baked beans jigsaw; the blank jigsaw. (In the past I've assembled a jigsaw which depicts three rainbow-coloured rings on a completely black background: in a row on one side, interlinked on the reverse. Since this is a double-sided jigsaw many of the pieces are completely black and have eight possible orientations...) These are the rational jigsaws: they are few and exceptional.