"The Pharisees came and began to question Jesus. To test him, they asked him for a sign from heaven. He sighed deeply and said, 'Why does this generation ask for a miraculous sign? I tell you the truth, no sign will be given to it.' Then he left them, got back into the boat and crossed to the other side." NIV translation
After this, the Jewish leaders tried to test Jesus again by asking for a miracle. He had just fed well over 4000 people and yet they asked for a sign. You can feel the sadness as Jesus let out a deep sigh of pity. He felt sorry for them because they could not see the signs that were right in front of them so they kept asking for more signs. It is much the same today as people look for spectacular signs of the end of the age instead of looking at the small signs around them.
"The disciples had forgotten to bring bread, except for one loaf they had with them in the boat. 'Be careful,' Jesus warned them. 'Watch out for the yeast of the Pharisees and that of Herod.' They discussed this with one another and said, 'It is because we have no bread.'" NIV translation
While they were in the boat, Jesus used the example of bread to teach His disciples. He warned them about yeast which is used to make bread fluffy and to taste good. It is a single cell organism that converts its food (sugar and starch) into carbon dioxide. The men would have been familiar with the removal of yeast (leaven) from their homes during the celebration of the Passover but were probably wondering why Jesus was telling them this now. They talked about it among themselves and could only come up with the fact that they did not have but one loaf of bread with them. They were still focused on the physical instead of spiritual matters.
We may be quick to criticize them as we can look back at the entire ministry of Christ and clearly see what He was talking about, but, they did not have all of the facts that we do. One thing that we can learn from this is that we must look to Jesus for the answers about spiritual matters and not the views of men.
"Aware of their discussion, Jesus asked them: 'Why are you talking about having no bread? Do you still not see or understand? Are your hearts hardened? Do you have eyes but fail to see, and ears but fail to hear? And don't you remember?'" NIV translation
Here, Jesus asks them five questions and we are reminded that, throughout the Bible, the number five is associated with grace and grace is a gift. With these questions, He is trying to get them to see past the human desire to work for the things of God. The yeast that He had warned them about was the teaching of the Pharisees that was trying to add the Law (works) to grace.