Cyril of Alexandria — as excerpted in the Catena Aurea on Luke 11:9-13

“In these words our Saviour gives us a very necessary piece of instruction. For often-times we rashly, from the impulse of pleasure, give way to hurtful desires. When we ask any such thing from God, we shall not obtain it. To shew this, He brings an obvious example from those things which are before our eyes, in our daily experience. For when thy son asks of thee bread, thou givest it him gladly, because he seeks a wholesome food. But when from want of understanding he asks for a stone to eat, thou givest it him not, but rather hinderest him from satisfying his hurtful desire. So that the sense may be, But which of you asking his father for bread, (which the father gives,) will he give him a stone? (that is, if he asked it.) There is the same argument also in the serpent and the fish; of which he adds, Or if he asks a fish, will he for a fish give him a serpent? And in like manner in the egg and scorpion, of which he adds, Or if he ask an egg, will he offer him a scorpion?”