The Pharisees and Sadducees came to Jesus and tested him by asking him to show them a sign from heaven. He replied, "When evening comes, you say, 'It will be fair weather, for the sky is red,' and in the morning, 'Today it will be stormy, for the sky is red and overcast.' You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times. A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." Jesus then left them and went away. When they went across the lake, the disciples forgot to take bread. "Be careful," Jesus said to them. "Be on your guard against the yeast of the Pharisees and Sadducees." They discussed this among themselves and said, "It is because we didn't bring any bread." Aware of their discussion, Jesus asked, "You of little faith, why are you talking among yourselves about having no bread? Do you still not understand? Don't you remember the five loaves for the five thousand, and how many basketfuls you gathered? Or the seven loaves for the four thousand, and how many basketfuls you gathered? How is it you don't understand that I was not talking to you about bread? But be on your guard against the yeast of the Pharisees and Sadducees." Then they understood that he was not telling them to guard against the yeast used in bread, but against the teaching of the Pharisees and Sadducees.