Matthew 16:12

12 Then the disciples understood what Jesus was talking about. They did not need to be careful about the yeast that they used in their bread. Instead, they must be careful about what the Pharisees and Sadducees were teaching. They must not accept it.

16:12People put yeast into flour and water to make bread. The yeast grows in the bread and it makes the bread become bigger. The Pharisees were teaching people wrong things. Their wrong teaching spread to more and more people.