The Brain Teaser: Scroll Down for your Solution
A woman was sitting in her hotel room when there was 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." He then went off down the corridor to the elevator. The woman went back into her room and phoned security to have the man apprehended. What made the woman so suspicious of the man.
She reasoned that if really thought that it was his room, he would have used his key instead of knocking on the door. In fact, he knocked in order to check whether anyone was in the room before using a pass key to enter and burgle the rooms.