I learned during some child development classes I took in college that you shouldn’t give your child the opportunity to lie to you, since it establishes a negative pattern of behavior and mistrust. Instead of confronting them by asking them if they’re doing the thing you know they’re doing, confront them by telling them you know they’re doing the thing and show them why you know. Sure, they may learn how to cover their tracks better as a result, but it eliminates a lot of arguing and prevents a cycle of lying.