You're a kid who seemed nice and innocent. In reality you were a kid who dealed with people who kill people. Although one night for fun, you decided to rat them out. The police were at the scene, and you told them the finger prints of the people who did it were on the knife they used. A few days later the police appeared at the door, your mother opened and the police were there to talk to you. Becuase you were, in fact, right about the knife. They needed to know how you knew that.