How did she Know?

So, Chad was totally joking around with Heidi and asked if she did anything bad at school. She said no. He then told her that she better fess up because her teacher called and told him what she had done, which was a total lie. Heidi responded with a question, "How did she know?" At this point, Chad and I laughing so hard and wondering what she is going to fess up to. It ended up that she accidentally pulled some kids hair while she was trying to grab her chair. What a naguhty girl, huh? Chad told he was kidding, but thanked her for telling the truth. Funny kids!