The disciples heard the word "leaven" and thought about bread. They had only one loaf, and their talk turned to what they lacked. Gill says Jesus knew their talk the way He knew the Pharisees' thoughts: "as he knew the thoughts and reasonings of the Scribes and Pharisees ... so he did those of his own disciples."

Each question goes a layer deeper. The first asks why bread is filling their minds. The second asks why they still do not see what He has shown them. Gill says "it might have been expected, that by a second miracle of the loaves, their understandings would have been more enlightened, and their faith increased." The third asks whether their hearts have grown hard, the very thing Mark said of them after the first feeding (Mark 6:52).

The disciples give no answer to these three questions. Jesus keeps pressing: "Having eyes, see ye not? and having ears, hear ye not? and do ye not remember?" (Mark 8:18). They speak only when He asks for numbers, answering "Twelve" and "Seven" (Mark 8:19-20). He closes with one more question: "How is it that ye do not understand?" (Mark 8:21). Matthew adds that at last they understood He meant "the doctrine of the Pharisees and of the Sadducees" (Matthew 16:12).

Leaven is yeast, a small lump of fermenting dough worked into new dough until it spreads through the whole batch. Barnes explains that Jesus was using it for the doctrines of the Pharisees, which were "insinuating, artful, plausible" and spread secretly through the mind. He adds that the Jews did not often use "leaven" for doctrine, which helps explain why the disciples missed the point. Mark names the leaven "of Herod" where Matthew names the Sadducees (Matthew 16:6). Barnes thinks it "not improbable" that Jesus warned against them all, so that "the evangelists speak the same thing."

The word "hardened" is the Greek poroo, which Thayer defines as "to cover with a thick skin, to harden by covering with a callus." Mark uses the same verb of the disciples after Jesus fed the five thousand (Mark 6:52). The word "reason" is dialogizomai, to talk something over or debate it.

These questions show what Jesus most wants from His followers. His questions are not about a forgotten loaf. They press on how men who had watched Him feed thousands, twice, could still reason about having no bread with Him in the ship. Matthew Henry puts it plainly: "Thus distrust of God makes Christ's disciples quarrel among themselves."

The question about hard hearts is striking because of where it lands. Just before this, the Pharisees demanded a sign and Jesus "sighed deeply in his spirit" (Mark 8:11-12). Now His own friends show a smaller version of the same blindness. Yet He does not leave them. He keeps asking questions, walks them back through what they saw, and then heals a blind man in stages (Mark 8:22-25), right before Peter finally sees who He is (Mark 8:29).