The Ancient Mariner at the Sermon on the Mount

There's an ancient Greek proverb, or saying, 'a socrpion for a perch' (anti perkees scorpion) that, the scholars tell us, found its way somehow into the Sermon on the Mount: Matthew 7:9-10: 'Or what man is there of you, whom if his son ask bread, will he give him a stone? Or if he ask a fish, will he give him a serpent?' I suppose, though, that we're entitled at least to wonder at the logic here. It's possible--isn't it?--to imagine a situation in which, though a son wants some bread, what he, and we, need is for him to finish laying that stone wall first; or in which the glass case he possesses is not watertight enough to serve as home for a fish, but with a few sticks and leaves would perfectly suit our gift to him of a snake. Of course, it's also possible to imagine son and father bickering ('I want a fish!' ... 'you'll take the snake and you'll like it!' ... 'never! a fish or nothing!' ... 'snake I say!') and the mother interceding, like the Ancient Mariner at the Sermon, with: 'here, a compromise: water-snakes.'