Wouldn't it be nice if people who told lies gave obvious signs that they were being untruthful?
1. If a person says one thing, then says another, and you ask them which it is, a person who is telling the truth will have a rational, logical explanation. A person who is lying will react. They may get overly angry, or they may ask, "What are you accusing me of!?"
This is an attempt to scare you away from the truth. If they accepted the truth, they wouldn't have made up a lie in the first place, so it's only logical that they will still try to keep you away from it.