Jesus wants His hearers to answer for themselves: no father would do that. This is His second picture, and it makes the first one sharper. A stone is useless to a hungry child, but a serpent could hurt him. Barnes points out that "a fish has some resemblance to a serpent; yet no parent would attempt to deceive his child in this." Gill says the same, that a serpent is "somewhat like a fish, especially an eel." So the question asks whether a father would trick his child, handing over something that looks like food but is harmful. The answer is obvious, and it sets up the point Jesus makes about God in Matthew 7:11.

No one needs to reply, because the answer is obvious: no father would do this. Jesus Himself draws the conclusion in the next breath: "how much more shall your Father which is in heaven give good things to them that ask him?" (Matthew 7:11). When the sermon ended, "the people were astonished at his doctrine" (Matthew 7:28).

Bread and fish were everyday food. Gill notes that "fish and bread are mentioned, because these were common food." A father asked for either one would be meeting his child's most ordinary need.

Luke records the same teaching on another occasion and adds a third picture: "Or if he shall ask an egg, will he offer him a scorpion?" (Luke 11:12). In each pair, the bad gift looks a little like the good one, which makes the idea of a father giving it even more unthinkable.

This small question is about the heart of God. Jesus has just urged His hearers to ask, seek, and knock (Matthew 7:7-8). Jesus answers any fear that God will give something hurtful by pointing to the most basic love they know, a father feeding his child. Barnes draws the lesson plainly: "God is better and kinder than the most tender earthly parents."

The question also shows how Jesus teaches. He does not just state that God is good. He lets His hearers reason it out from what they already know, so the conclusion becomes their own (Matthew 7:11).