Jesus wanted His disciples to trust that God will answer them with good. So He appealed to something every father knows. Gill says, "Our Lord illustrates and confirms what he had said before by an instance common among men." A hungry son asks for bread or a fish. No father in the room would hand him a stone or a snake instead. Barnes, on the parallel in Matthew 7:9-10, puts it simply: a father "would not give him a stone instead of bread, or a serpent instead of a fish." The two questions press the same point from two everyday meals.

The answer to both questions is so obvious that no one needed to say it: of course not. Jesus Himself drew the conclusion two verses later: "If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?" (Luke 11:13).

Bread and fish were basic food, the kind of thing a son would ask for at a meal. Gill notes that the relation between father and son "is natural, and it is very near," and that it was usual for a son to ask bread of his father when hungry.

The KJV and the ESV differ in this verse. The KJV, following the Textus Receptus (the Greek text behind the KJV), includes both the bread and stone and the fish and serpent. The ESV follows Greek editions that have only the fish and serpent. Matthew's account has both pairs (Matthew 7:9-10).

Jesus teaches about God by pointing to the best instincts of an earthly father. If a father would never trick his hungry child, God will not answer His children's prayers with something harmful disguised as good.

This is an argument from the lesser to the greater, completed in Luke 11:13. Barnes, on the same teaching in Matthew, writes that "God is better and kinder than the most tender earthly parents; and with what confidence, therefore, may we come as his children, and ask what we need!" Jesus wanted His disciples to pray from that confidence.