A man was just about to put his key in the door when he heard his wife scream "No Charlie, don't do it!" The man walked in his house to find his wife dead on the floor next to a doctor, a lawyer and a police officer. None of them had name tags and the man did not know who any of them were. The only thing the man knew is that the police officer did it.
How did he know?
His wife yelled, "No Charlie." The doctor and lawyer were both women, so the police officer had to be Charlie.