"Watch your mouth!" Did your parents ever have to say that to you?  Maybe you said something hurtful or sinful. In the 9th commandment, God says: "You shall not give false testimony against your neighbor.". Jesus put up with a lot of "false testimony", especially in this story. He uses the situation to talk about speech.  As with other commandments, Jesus gets at the attitude as well as the action, the heart as well as the mouth.
Scripture: Matthew 12:22-37