When the disciples arrived on the other side of the lake, they had forgotten to bring bread.
Jesus said to them, "Watch out and be on your guard for the yeast of the Pharisees and Sadducees."
They discussed this among themselves and said, "We didn't bring any bread."
Jesus knew what they were discussing and said, "You people of weak faith! Why are you discussing among yourselves the fact that you don't have any bread?9 Don't you understand yet? Don't you remember the five loaves that fed the five thousand and how many baskets of leftovers you gathered?10 And the seven loaves that fed the four thousand and how many large baskets of leftovers you gathered?11 Don't you know that I wasn't talking about bread? But be on your guard for the yeast of the Pharisees and Sadducees."
Then they understood that he wasn't telling them to be on their guard for yeast used in making bread. No, he was telling them to watch out for the teaching of the Pharisees and Sadducees.