Jesus knows that some of us will hear the promise of verse 8 and still pull back, so He does what He so often does: He paints a picture. In Galilee, the small round loaves of bread looked remarkably like the smooth stones along the shore, and some of the fish in the Sea of Galilee were eel-like, easily mistaken for a snake. R. T. France notes that the rhetorical question depicts what should be an unthinkable response to a child’s request: not merely denying food, but the cynical substitution of something superficially similar that is either useless, a stone, or positively harmful, a serpent.
The form of the question in the Greek expects the answer, of course not. No parent does this. When your child looks up from the table and asks for bread, you do not hand them a rock from the driveway. Even the most flawed parents feed their children. Jesus builds His argument on something so universally true that no one in the crowd could object.
And this is the point: if the unthinkable response is unthinkable even for us, how much more unthinkable is it for God? Your Father does not play cruel jokes. He does not answer requests with counterfeits. When you come to Him hungry, He does not mock your hunger. Whatever else you believe about your prayers that seem to hang unanswered, you may not believe this: that God has handed you a stone. He has not. He never will.