One day Charlie was found under a cliff dead. The police called Charlie's brother Sean who had witnessed the whole thing. This is what Sean said to the police: "I was taking a walk along the cliff, to find Charlie and his girlfriend fighting. I didn't want to interrupt so I stood behind a tree. Charlie was wearing a big leather jacket and had on gloves and jeans. He was also wearing black boots. That's when I saw Charlie pick up a rock, threatening to hit his girlfriend with it. That's when I approached him to tell him to stop. I must have startled him because he started running right off the cliff". The police immediately investigated where Charlie was fighting with his girlfriend. The police found the rock charlie was about to throw at his girlfriend, with fingerprints on it. Without scanning the fingerprints, The police immediately arrested Sean. Why?