It's a sad truth that the moment a child has learned how to talk and communicate, it's not far away from learning how to lie. Children are far smarter than most people give them credit for, and they're more than happy to use ambiguity to their own advantage. Trying to persuade a child to brush their teeth is a regular battle for a lot of parents. If you show them the hint of a way out of it, they'll take it.
If you're asking your child if they've cleaned their teeth, their bedroom or themselves, make sure you specify exactly when you expect them to have done it. The same applies to chores, homework or just about anything else. If you ask them a vague question and they give you a vague answer they haven't lied to you, they've just stepped through the loophole you provided to them.