Riddle: A lady sat in her hotel room when there is a knock at the door. She opened the door to see a man whom she had never seen before. He said, "Oh I'm sorry, I have made a mistake, I thought this was my room." Then, he left. The woman went back into her room and phoned security. What made the woman so suspicious of the man?
Answer: You don't knock on your own hotel room.
Source: https://riddles.com/4543