My Preteen has started lying. She had a project and told me that she was working on it. That a milestone was completed at school or at her father's house. Her best friend ratted her out the Friday before it was due so I grounded her all weekend to get it done and helped. The remaining 10% she said that she was completing on the computer but was actually searching youtube and would switch to a research screen whenever I walked in. So 5 weeks of lies over one project. I am fed up. I told her how dissapointed I was and she looked guilty only to lie and say she was working on it the next day but wasn't.
How do I stop the lying?