A carelessly built story will sink as certainly as a carelessly built boat, so I entirely agree with your point about Jaws. But this is about story engineering, it’s the deus ex machina resolution: of course we’d feel cheated by a story constructed in this way. It needn’t be about the magic wand: the story would sink just as surely if there were some psychological inconsistency – if Schneider’s character had suddenly become suicidally depressed and thrown himself into the jaws of the shark – or a narrative inconsistency – if he had lost his nerve, avoided the confrontation altogether, and sailed off to a retirement in Fiji.

Equally you could have a well designed story about a psychologically consistent character living within an well-constructed internally consistent alternative reality. You could have all this with a tedious character and a dull story which wouldn’t be remotely enjoyable.

In terms of the physics of storytelling, the forces in the willing suspension of disbelief are our disbelief itself – this is a resistive force, like friction – but also our will to believe. It’s the strength of the latter that is keeping the fish in the air, and I think it would be diminished by tinkering with the genetics of the fish.

Air fish are the product of a childish imagination, before the child had understood about the mechanics of weight and thrust and the relative density of water and air, but after they had seen how beautiful fish are and how gracefully the move.

It’s a bit like geneticists discovering some new species with an incredible ability to live somewhere it’s poorly adapted for and without understanding why it survives, trying to engineer adaptations to help it only to find each modified specimen dying. A story engineer trying to do this with the fictional air fish (I imagine a story engineer as a bit like a kind of celestial toymaker – no, like the master of the land of fiction) would find the specimens with the enlarged fins flapping around helplessly on the pavement while the poorly adapted fish glide round the lamppost blissfully unaware of the laws of physics they’re breaking. Why? Because the less the modified fish resemble the childish dream, the weaker the force of our will to suspend disbelief. Anyway, just how much would you have to modify those fish to actually make me believe they could really fly? I’d believe in porcine elevation first.