Jesus asked this to draw His hearers into thinking with Him. The word "we" invites them to search for the right picture together. Barnes says the question shows "the great solicitude which Jesus had to adapt his instructions to the capacity of his disciples," and that "He sought out the most plain and striking illustrations."

Gill adds two things. Such questions were a familiar way to begin a parable: "It was usual with the Jewish doctors, when about to illustrate anything in a parabolical way to begin with such like questions." And Jesus' aim was to give His disciples clearer ideas of "the success of the Gospel, and the usefulness of their ministration of it, for their encouragement."

Jesus answered His own question: "It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth: But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it" (Mark 4:31-32). Mark adds that "with many such parables spake he the word unto them, as they were able to hear it" (Mark 4:33).

In the Greek text behind the KJV, the second question uses a noun and verb from the same root: parabolē, "comparison" or parable, and paraballō, "compare." It reads almost like "with what parable shall we parable it?" Thayer defines parabolē first as "a placing of one thing by the side of another." The ESV follows a slightly different Greek text: "what parable shall we use for it?"

The mustard plant of that region grew large. Barnes, in his notes on Matthew 13:31, says it "was very different from that which is known among us," and quotes the traveler Thomson: "I have seen this plant on the rich plain of Akkar as tall as the horse and his rider."

The kingdom of God starts small and becomes great. Jesus placed this picture right after the seed that grows "he knoweth not how" (Mark 4:27), where "the earth bringeth forth fruit of herself" (Mark 4:28). Barnes draws from this that such growth "is by the power of God," not by the power of man. Matthew Henry applies it to each believer: "The work of grace in the soul is, at first, but the day of small things; yet it has mighty products even now."

The question itself shows something of Jesus' heart as a teacher. He spoke "as they were able to hear it" (Mark 4:33), searching for pictures His hearers could hold. Matthew and Luke record the same parable (Matthew 13:31-32; Luke 13:18-19), and in Luke, Jesus asks a similar question again before the parable of the leaven (Luke 13:20-21).